The Ancient Life History of the Earth Part 7

[Ill.u.s.tration: Fig. 43.--_Zaphrentis Stokesi_, a simple "cup-coral,"

Upper Silurian, Canada. (After Billings.)]

[Ill.u.s.tration: Fig. 44.--Upper surface of a ma.s.s of _Strombodes pentagonus_. Upper Silurian, Canada. (After Billings.)]

In the general details of their structure, the Lower Silurian Corals do not differ from the ordinary Corals of the present day. The latter, however, have the vertical calcareous plates of the coral ("septa") arranged in multiples of six or five; whereas the former have these structures arranged in multiples of four, and often showing a cross-like disposition. For this reason, the common Lower Silurian Corals are separated to form a distinct group under the name of _Rugose_ Corals or _Rugosa_.

They are further distinguished by the fact that the cavity of the coral ("visceral chamber") is usually subdivided by more or less numerous horizontal calcareous plates or part.i.tions, which divide the coral into so many tiers or storeys, and which are known as the "tabulae" (fig. 45).

[Ill.u.s.tration: Fig. 45.--_Columnaria alveolata_, a Rugose compound coral, with imperfect septa, but having the corallites part.i.tioned off into storeys by "tabulae." Lower Silurian, Canada. (After Billings.)]

In addition to the Rugose Corals, the Lower Silurian rocks contain a number of curious compound corals, the tubes of which have either no septa at all or merely rudimentary ones, but which have the transverse part.i.tions or "tabulae" very highly developed.

These are known as the _Tabulate Corals_; and recent researches on some of their existing allies (such as _Heliopora_) have shown that they are really allied to the modern Sea-pens, Organ-pipe Corals, and Red Coral, rather than to the typical stony Corals.

Amongst the characteristic Rugose Corals of the Lower Silurian may be mentioned species belonging to the genera _Columnaria, Favistella, Streptelasma_, and _Zaphrentis_; whilst amongst the "Tabulate" Corals, the princ.i.p.al forms belong to the genera _Choetetes, Halysites_ (the Chain-coral), _Constellaria_, and _Heliolites_. These groups of the Corals, however, attain a greater development at a later period, and they will be noticed more particularly hereafter.

[Ill.u.s.tration: Fig. 46.--Group of Cystideans. A, _Caryocrinus ornatus_,[13] Upper Silurian, America; B, _Pleurocyst.i.tes squamosus_, showing two short "arms," Lower Silurian, Canada; C, _Pseudocrinus bifasciatus_, Upper Silurian, England; D, _Lepadocrinus Gebhartii_, Upper Silurian, America. (After Hall, Billings, and Salter.)]

[Footnote 13: The genus _Caryocrinus_ is sometimes regarded as properly belonging to the _Crinoids_, but there seem to be good reasons for rather considering it as an abnormal form of _Cystidean_.]

Pa.s.sing onto higher animals, we find that the cla.s.s of the _Echinodermata_ is represented by examples of the Star-fishes (_Asteroidea_), the Sea-lilies (_Crinoidea_), and the peculiar extinct group of the Cystideans (_Cystoidea_), with one or two of the Brittle-stars (_Ophiuroidea_)--the Sea-urchins (_Echinoidea_) being still wanting. The Crinoids, though in some places extremely numerous, have not the varied development that they possess in the Upper Silurian, in connection with which their structure will be more fully spoken of. In the meanwhile, it is sufficient to note that many of the calcareous deposits of the Lower Silurian are strictly ent.i.tled to the name of "Crinoidal limestones,"

being composed in great part of the detached joints, and plates, and broken stems, of these beautiful but fragile organisms (see fig. 12). Allied to the Crinoids are the singular creatures which are known as _Cystideans_ (fig. 46). These are generally composed of a globular or ovate body (the "calyx"), supported upon a short stalk (the "column"), by which the organism was usually attached to some foreign body. The body was enclosed by closely-fitting calcareous plates, accurately jointed together; and the stem was made up of numerous distinct pieces or joints, flexibly united to each other by membrane. The chief distinction which strikes one in comparing the Cystideans with the Crinoids is, that the latter are always furnished, as will be subsequently seen, with a beautiful crown of branched and feathery appendages, springing from the summit of the calyx, and which are composed of innumerable calcareous plates or joints, and are known as the "arms." In the Cystideans, on the other hand, there are either no "arms" at all, or merely short, unbranched, rudimentary arms. The Cystideans are princ.i.p.ally, and indeed nearly exclusively, Silurian fossils; and though occurring in the Upper Silurian in no small numbers, they are pre-eminently characteristic of the Llandeilo-Caradoc period of Lower Silurian time. They commenced their existence, so far as known, in the Upper Cambrian; and though examples are not absolutely unknown in later periods, they are pre-eminently characteristic of the earlier portion of the Palaeozoic epoch.

[Ill.u.s.tration: Fig. 47.--Lower Silurian Crustaceans. a, _Asaphus tyrannus_, Upper Llandeilo; b. _Ogygia Buchii_, Upper Llandeilo; c, _Trinucleus concentricus_, Caradoc; d, _Caryocaris Wrightii_, Arenig (Skiddaw Slates); e, _Beyrichia complicata_, natural size and enlarged, Upper Llandeilo and Caradoc; f, _Primitia strangulata_, Caradoc: g. Head-s.h.i.+eld of _Calymene Blumenbachii_, var.

_brevicapitata_, Caradoc; h, Head-s.h.i.+eld of _Triarthrus Becki_ (Utica Slates), United States: i, s.h.i.+eld of _Leperditia Canadensis_, var. _Josephiana_, of the natural size, Trenton Limestone, Canada; j, The same, viewed from the front. (After Salter, M'Coy, Rupert Jones, and Dana.)]

The Ringed Worms (_Annelides_) are abundantly represented in the Lower Silurian, but princ.i.p.ally by tracks and burrows similar in essential respects to those which occur so commonly in the Cambrian formation, and calling for no special comment. Much more important are the _Articulate_ animals, represented as heretofore, wholly by the remains of the aquatic group of the _Crustaceans_.

Amongst these are numerous little bivalved forms--such as species of _Primitia_ (fig. 47, f), _Beyrichia_ (fig. 47, e), and _Leperditia_ (fig. 47, i and j). Most of these are very small, varying from the size of a pin's head up to that of a hemp seed; but they are sometimes as large as a small bean (fig. 47, i), and they are commonly found in myriads together in the rock. As before said, they belong to the same great group as the living Water-fleas (_Ostracoda_). Besides these, we find the pod-shaped head-s.h.i.+elds of the shrimp-like Phyllopods--such as _Caryocaris_ (fig. 47, d) and _Ceratiocaris_. More important, however, than any of these are the _Trilobites_, which may be considered as attaining their maximum development in the Lower Silurian. The huge _Paradoxides_ of the Cambrian have now disappeared, and with them almost all the princ.i.p.al and characteristic "primordial"

genera, save _Olenus_ and _Agnostus_. In their place we have a great number of new forms--some of them, like the great _Asaphus tyrannus_ of the Upper Llandeilo (fig. 47, a), attaining a length of a foot or more, and thus hardly yielding in the matter of size to their ancient rivals. Almost every subdivision of the Lower Silurian series has its own special and characteristic species of Trilobites; and the study of these is therefore of great importance to the geologist. A few widely-dispersed and characteristic species have been here figured (fig. 47); and the following may be considered as the princ.i.p.al Lower Silurian genera--_Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus, Lichas, Illoenus, aeglina, Harpes, Remopleurides, Phacops, Acidaspis_, and _Homalonotus_, a few of them pa.s.sing upwards under new forms into the Upper Silurian.

Coming next to the _Mollusca_, we find the group of the Sea-mosses and Sea-mats (_Polyzoa_) represented now by quite a number of forms.

Amongst these are examples of the true Lace-corals (_Retepora_ and _Fenestella_), with their netted fan-like or funnel-shaped fronds; and along with these are numerous delicate encrusting forms, which grew parasitically attached to sh.e.l.ls and corals (_Hippothoa, Alecto_, &c.); but perhaps the most characteristic forms belong to the genus _Ptilodictya_ (figs. 48 and 49). In this group the frond is flattened, with thin striated edges, sometimes sword-like or scimitar-shaped, but often more or less branched; and it consists of two layers of cells, separated by a delicate membrane, and opening upon opposite sides. Each of these little chambers or "cells" was originally tenanted by a minute animal, and the whole thus const.i.tuted a compound organism or colony.

[Ill.u.s.tration: Fig. 48.--_Ptilodictya falciformis_. a, Small specimen of the natural size; b, Cross-section, showing the shape of the frond; c, Portion of the surface, enlarged. Trenton Limestone and Cincinnati Group, America. (Original.)]

[Ill.u.s.tration: Fig. 49.--A, _Ptilodictya acuta_; B, _Ptilodictya Schafferi_. a, Fragment, of the natural size; b, Portion, enlarged to show the cells. Cincinnati Group of Ohio and Canada.

(Original.)]

[Ill.u.s.tration: Fig. 50.--Lower Silurian Brachiopods. a and a', _Orthis biforata_, Llandeilo-Caradoc, Britain and America: b, _Orthis flabellulum_, Caradoc, Britain: c, _Orthis subquadrata_, Cincinnati Group, America; c', Interior of the dorsal valve of the same: d, _Strophomena deltoidea_, Llandeilo-Caradoc, Britain and America. (After Meek, Hall, and Salter.)]

The Lamp-sh.e.l.ls or _Brachiopods_ are so numerous, and present such varied types, both in this and the succeeding period of the Upper Silurian, that the name of "Age of Brachiopods" has with justice been applied to the Silurian period as a whole. It would be impossible here to enter into details as to the many different forms of Brachiopods which present themselves in the Lower Silurian deposits; but we may select the three genera _Orthis, Strophomena_, and _Leptoena_ for ill.u.s.tration, as being specially characteristic of this period, though not exclusively confined to it.

The numerous sh.e.l.ls which belong to the extensive and cosmopolitan genus _Orthis_ (fig. 50, a, b, c, and fig. 51, c and d), are usually more or less transversely-oblong or subquadrate, the two valves (as more or less in all the Brachiopods) of unequal sizes, generally more or less convex, and marked with radiating ribs or lines. The valves of the sh.e.l.l are united to one another by teeth and sockets, and there is a straight hinge-line. The beaks are also separated by a distinct s.p.a.ce ("hinge-area"), formed in part by each valve, which is perforated by a triangular opening, through which, in the living condition, pa.s.sed a muscular cord attaching the sh.e.l.l to some foreign object. The genus _Strophomena_ (fig. 50, d, and 51, a and b) is very like _Orthis_ in general character; but the sh.e.l.l is usually much flatter, one or other valve often being concave, the hinge-line is longer, and the aperture for the emission of the stalk of attachment is partially closed by a calcareous plate. In _Leptoena_, again (fig. 51, e), the sh.e.l.l is like _Strophomena_ in many respects, but generally comparatively longer, often completely semicircular, and having one valve convex and the other valve concave. Amongst other genera of Brachiopods which are largely represented in the Lower Silurian rocks may be mentioned _Lingula, Crania, Discina, Trematis, Siphonotreta, Acrotreta, Rhynchonella_, and _Athyris_; but none of these can claim the importance to which the three previously-mentioned groups are ent.i.tled.

[Ill.u.s.tration: Fig. 51.--Lower Silurian Brachiopods, a, _Strophomena alternata_, Cincinnati Group, America; b, _Strophomena filitexta, Trenton and Cincinnati Groups, America; c, _Orthis testudinaria_, Caradoc, Europe, and America; d, d', _Orthis plicateila_, Cincinnati Group, America; e, e', e'', _Leptoena sericea_, Llandeilo and Caradoc, Europe and America. (After Meek, Hall, and the Author.)]

The remaining Lower Silurian groups of _Mollusca_ can be but briefly glanced at here. The Bivalves (_Lamellibranchiata_) find numerous representatives, belonging to such genera as _Modiolopsis, Ctenodonta, Orthonota, Paloearca, Lyrodesma, Ambonychia_, and _Cleidophorus_. The Univalves (_Gasteropoda_) are also very numerous, the two most important genera being _Murchisonia_ (fig. 52) and _Pleurotomaria_. In both these groups the outer lip of the sh.e.l.l is notched; but the sh.e.l.l in the former is elongated and turreted, whilst in the latter it is depressed. The curious oceanic Univalves known as the _Heteropods_ are also very abundant, the princ.i.p.al forms belonging to _Bellerophon_ and _Maclurea_. In the former (fig. 53) there is a symmetrical convoluted sh.e.l.l, like that of the Pearly Nautilus in shape, but without any internal part.i.tions, and having the aperture often expanded and notched behind. The species of _Maclurea_ (fig. 54) are found both in North America and in Scotland, and are exclusively confined to the Lower Silurian period, so far as known. They have the sh.e.l.l coiled into a flat spiral, the mouth being furnished with a very curious, thick, and solid lid or "operculum." The Lower Silurian _Pteropods_, or "Winged snails," are numerous, and belong princ.i.p.ally to the genera _Theca, Conularia_, and _Tentaculites_, the last-mentioned of these often being extremely abundant in certain strata.

[Ill.u.s.tration: Fig. 52.--_Murchisonia gracilis_, Trenton Limestone, America. (After Billings.)]

[Ill.u.s.tration: Fig. 53.--Different views of _Bellerophon Argo_, Trenton Limestone, Canada. (After Billings.)]

[Ill.u.s.tration: Fig. 54.--Different views of _Maclurea crenulata_, Quebec Group, Newfoundland. (After Billings.)]

[Ill.u.s.tration: Fig. 55.--Fragment of _Orthoceras crebriseptum_, Cincinnati Group, North America, of the natural size. The lower figure section showing the air-chambers, and the form and position of the siphuncle. (After Billings.)]

[Ill.u.s.tration: Fig. 56.--[14] Restoration of Orthoceras, the sh.e.l.l being supposed to be divided vertically, and only its upper part being shown. a, Arms; f, Muscular tube ("funnel") by which water is expelled from the mantle-chamber; c, Air-chambers; s, Siphuncle.]

[Footnote 14: This ill.u.s.tration is taken from a rough sketch made by the author many years ago, but he is unable to say from what original source it was copied.]

Lastly, the Lower Silurian Rocks have yielded a vast number of chambered sh.e.l.ls, referable to animals which belong to the same great division as the Cuttle-fishes (the _Cephalopoda_), and of which the Pearly Nautilus is the only living representative at the present day. In this group of _Cephalopods_ the animal possesses a well-developed external sh.e.l.l, which is divided into chambers by sh.e.l.ly part.i.tions ("septa"). The animal lives in the last-formed and largest chamber of the sh.e.l.l, to which it is organically connected by muscular attachments. The head is furnished with long muscular processes or "arms," and can be protruded from the mouth of the sh.e.l.l at will, or again withdrawn within it. We learn, also, from the Pearly Nautilus, that these animals must have possessed two pairs of breathing organs or "gills;" hence all these forms are grouped together under the name of the "Tetrabranchiate" Cephalopods (Gr. _tetra_, four; _bragchia_, gill). On the other hand, the ordinary Cuttle-fishes and Calamaries either possess an internal skeleton, or if they have an external sh.e.l.l, it is not chambered; their "arms" are furnished with powerful organs of adhesion in the form of suckers; and they possess only a single pair of gills. For this last reason they are termed the "Dibranchiate" Cephalopods (Gr. _dis_, twice; _bragchia_, gill). No trace of the true Cuttle-fishes has yet been found in Lower Silurian deposits; but the Tetrabranchiate group is represented by a great number of forms, sometimes of great size. The princ.i.p.al Lower Silurian genus is the well-known and widely-distributed _Orthoceras_ (fig. 55). The sh.e.l.l in this genus agrees with that of the existing _Pearly Nautilus_, in consisting of numerous chambers separated by sh.e.l.ly part.i.tions (or septa), the latter being perforated by a tube which runs the whole length of the sh.e.l.l after the last chamber, and is known as the "siphuncle" (fig. 56, s). The last chamber formed is the largest, and in it the animal lives. The chambers behind this are apparently filled with some gas secreted by the animal itself; and these are supposed to act as a kind of float, enabling the creature to move with ease under the weight of its sh.e.l.l. The various air-chambers, though the siphuncle pa.s.ses through them, have no direct connection with one another; and it is believed that the animal has the power of slightly altering its specific gravity, and thus of rising or sinking in the water by driving additional fluid into the siphuncle or partially emptying it.

The _Orthoceras_ further agrees with the Pearly Nautilus in the fact that the part.i.tions or septa separating the different air-chambers are simple and smooth, concave in front and convex behind, and devoid of the elaborate lobation which they exhibit in the Ammonites; whilst the siphuncle pierces the septa either in the centre or near it. In the Nautilus, however, the sh.e.l.l is coiled into a flat spiral; whereas in _Orthoceras_ the sh.e.l.l is a straight, longer or shorter cone, tapering behind, and gradually expanding towards its mouth in front. The chief objections to the belief that the animal of the _Orthoceras_ was essentially like that of the Pearly Nautilus are--the comparatively small size of the body-chamber, the often contracted aperture of the mouth, and the enormous size of some specimens of the sh.e.l.l.

Thus, some _Orthocerata_ have been discovered measuring ten or twelve feet in length, with a diameter of a foot at the larger extremity. These colossal dimensions certainly make it difficult to imagine that the comparatively small body-chamber could have held an animal large enough to move a load so ponderous as its own sh.e.l.l. To some, this difficulty has appeared so great that they prefer to believe that the _Orthoceras_ did not live in its sh.e.l.l at all, but that its sh.e.l.l was an internal skeleton similar to what we shall find to exist in many of the true Cuttle-fishes. There is something to be said in favour of this view, but it would compel us to believe in the existence in Lower Silurian times of Cuttle-fishes fully equal in size to the giant "Kraken" of fable. It need only be added in this connection that the Lower Silurian rocks have yielded the remains of many other Tetrabranchiate Cephalopods besides _Orthoceras_. Some of these belong to _Cyrtoceras_, which only differs from _Orthoceras_ in the bow-shaped form of the sh.e.l.l; others belong to _Phragmoceras_, _Lituites_, &c.; and, lastly; we have true _Nautili_, with their spiral sh.e.l.ls, closely resembling the existing Pearly Nautilus.

Whilst all the sub-kingdoms of the Invertebrate animals are represented in the Lower Silurian rocks, no traces of Vertebrate animals have ever been discovered in these ancient deposits, unless the so-called "Conodonts" found by Pander in vast numbers in strata of this age [15] in Russia should prove to be really of this nature. These problematical bodies are of microscopic size, and have the form of minute, conical, tooth-shaped spines, with sharp edges, and hollow at the base. Their original discoverer regarded them as the h.o.r.n.y teeth of fishes allied to the Lampreys; but Owen came to the conclusion that they probably belonged to Invertebrates. The recent investigation of a vast number of similar but slightly larger bodies, of very various forms, in the Carboniferous rocks of Ohio, has led Professor Newberry to the conclusion that these singular fossils really are, as Pander thought, the teeth of Cyclostomatous fishes. The whole of this difficult question has thus been reopened, and we may yet have to record the first advent of Vertebrate animals in the Lower Silurian.

[Footnote 15: According to Pander, the "Conodonts" are found not only in the Lower Silurian beds, but also in the "Ungulite Grit"

(Upper Cambrian), as well as in the Devonian and Carboniferous deposits of Russia. Should the Conodonts prove to be truly the remains of fishes, we should thus have to transfer the first appearance of vertebrates to, at any rate, as early a period as the Upper Cambrian.]

CHAPTER X.

THE UPPER SILURIAN PERIOD.

Having now treated of the Lower Silurian period at considerable length, it will not be necessary to discuss the succeeding group of the _Upper Silurian_ in the same detail--the more so, as with a general change of _species_ the Upper Silurian animals belong for the most part to the same great types as those which distinguish the Lower Silurian. As compared, also, as regards the total bulk of strata concerned, the thickness of the Upper Silurian is generally very much below that of the Lower Silurian, indicating that they represent a proportionately shorter period of time. In considering the general succession of the Upper Silurian beds, we shall, as before, select Wales and America as being two regions where these deposits are typically developed.

In Wales and its borders the general succession of the Upper Silurian rocks may be taken to be as follows, in ascending order (fig. 57):--

(1) The base of the Upper Silurian series is const.i.tuted by a series of arenaceous beds, to which the name of "May Hill Sandstone"

was applied by Sedgwick. These are succeeded by a series of greenish-grey or pale-grey slates ("Tarannon Shales"), sometimes of great thickness; and these two groups of beds together form what may be termed the "_May Hill Group_" (Upper Llandovery of Murchison). Though not very extensively developed in Britain, this zone is one very well marked by its fossils; and it corresponds with the "Clinton Group" of North America, in which similar fossils occur. In South Wales this group is clearly unconformable to the highest member of the subjacent Lower Silurian (the Llandovery group); and there is reason to believe that a similar, though less conspicuous, physical break occurs very generally between the base of the Upper and the summit of the Lower Silurian.

(2) The _Wenlock Group_ succeeds the May Hill group, and const.i.tutes the middle member of the Upper Silurian. At its base it may have an irregular limestone ("Woolhope Limestone"), and its summit may be formed by a similar but thicker calcareous deposit ("Wenlock Limestone"); but the bulk of the group is made up of the argillaceous and shaly strata known as the "Wenlock Shale." In North Wales the Wenlock group is, represented by a great acc.u.mulation of flaggy and gritty strata (the "Denbighs.h.i.+re Flags and Grits"), and similar beds (the "Coniston Flags" and "Coniston Grits") take the same place in the north of England.

(3) The _Ludlow Group_ is the highest member of the Upper Silurian, and consists typically of a lower arenaceous and shaly series (the "Lower Ludlow Rock") a middle calcareous member (the "Aymestry Limestone"), and an upper shaly and sandy series (the "Upper Ludlow Rock" and "Downton Sandstone"). At the summit, or close to the summit, of the Upper Ludlow, is a singular stratum only a few inches thick (varying from an inch to a foot), which contains numerous remains of crustaceans and fishes, and is well known under the name of the "bone-bed." Finally, the Upper Ludlow rock graduates invariably into a series of red sandy deposits, which, when of a flaggy character, are known locally as the "Tile-stones."

These beds are probably to be regarded as the highest member of the Upper Silurian; but they are sometimes looked upon as pa.s.sage-beds into the Old Red Sandstone, or as the base of this formation. It is, in fact, apparently impossible to draw any actual line of demarcation between the Upper Silurian and the overlying deposits of the Devonian or Old Red Sandstone series.

Both in Britain and in America the Lower Devonian beds repose with perfect conformity upon the highest Silurian beds, and the two formations appear to pa.s.s into one another by a gradual and imperceptible transition.

The Upper Silurian strata of Britain vary from perhaps 3000 or 4000 feet in thickness up to 8000 or 10,000 feet. In North America the corresponding series, though also variable, is generally of much smaller thickness, and may be under 1000 feet. The general succession of the Upper Silurian deposits of North America is as follows:--

(1) _Medina Sandstone_.--This const.i.tutes the base of the Upper Silurian, and consists of sandy strata, singularly devoid of life, and pa.s.sing below in some localities into a conglomerate ("Oneida Conglomerate"), which is stated to contain pebbles derived from the older beds, and which would thus indicate an unconformity between the Upper and Lower Silurian.

(2) _Clinton Group_.--Above the Medina sandstone are beds of sandstone and shale, sometimes with calcareous bands, which const.i.tute what is known as the "Clinton Group." The Medina and Clinton groups are undoubtedly the equivalent of the "May Hill Group" of Britain, as shown by the ident.i.ty of their fossils.

[Ill.u.s.tration: Fig. 57. GENERALIZED SECTION OF THE UPPER SILURIAN STRATA OF WALES AND SHROPs.h.i.+RE.]

(3) _Niagara Group_.--This group consists typically of a series of argillaceous beds ("Niagara Shale") capped by limestones ("Niagara Limestone"); and the name of the group is derived from the fact that it is over limestones of this age that the Niagara river is precipitated to form the great Falls. In places the Niagara group is wholly calcareous, and it is continued upwards into a series of marls and sandstones, with beds of salt and ma.s.ses of gypsum (the "Salina Group"), or into a series of magnesian limestones ("Guelph Limestones"). The Niagara group, as a whole, corresponds unequivocally with the Wenlock group of Britain.

(4) _Lower Helderberg Group_.--The Upper Silurian period in North America was terminated by the deposition of a series of calcareous beds, which derive the name of "Lower Helderberg" from the Helderberg mountains, south of Albany, and which are divided into several zones, capable of recognition by their fossils, and known by local names (Tentaculite Limestone, Water-lime, Lower Pentamerus Limestone, Delthyris Shaly Limestone, and Upper Pentamerus Limestone). As a whole, this series may be regarded as the equivalent of the Ludlow group of Britain, though it is difficult to establish any precise parallelism. The summit of the Lower Heiderberg group is const.i.tuted by a coa.r.s.e-grained sandstone (the "Oriskany Sandstone"), replete with organic remains, which have to a large extent a Silurian _facies_. Opinions differ as to whether this sandstone is to be regarded as the highest bed of the Upper Silurian or the base of the Devonian. We thus see that in America, as in Britain, no other line than an artificial one can be drawn between the Upper Silurian and the overlying Devonian.

As regards the _life_ of the Upper Silurian period, we have, as before, a number of so-called "Fucoids," the true vegetable nature of which is in many instances beyond doubt. In addition to these, however, we meet for the first time, in deposits of this age, with the remains of genuine land-plants, though our knowledge of these is still too scanty to enable us to construct any detailed picture of the terrestrial vegetation of the period.