Part 52 (1/2)

In the same grey paving-stones and coa.r.s.e roofing-slates, in which the _Cephalaspis_ occurs, in Forfars.h.i.+re and Kincardines.h.i.+re, the remains of marine plants or fucoids abound. They are frequently accompanied by groups of hexagonal, or nearly hexagonal markings, which consist of small flattened carbonaceous bodies, placed in a slight depression of the sandstone or shale. (See figs. 397 and 398.) They much resemble in form the sp.a.w.n of the recent Natica (see fig. 399.), in which the eggs are arranged in a thin layer of sand, and seem to have acquired a polygonal form by pressing against each other. The substance of the egg, if fossilized, might give rise to small pellicles of carbonaceous matter.

[Ill.u.s.tration: Fig. 399. Fragment of sp.a.w.n of British species of _Natica_.]

These fossils I have met with, both to the north of Strathmore, in the vertical shale beneath the conglomerate, and in the same beds in the Sidlaw hills, at all the points where fig. 4. is introduced in the section, p. 48.

[Ill.u.s.tration: Fig. 400. _Pterichthys_, Aga.s.siz; upper side, showing mouth; as restored by H. Miller.[345-A]]

Beds of red shale and red sandstone, sometimes a.s.sociated with pudding-stone (older than No. 3., fig. 62. p. 48.), and dest.i.tute of organic remains, separate, in the region of Strathmore, the above-described fossiliferous strata from the older crystalline rocks of the Grampians.

But, in the north of Scotland, we find, at the base of the Old Red, other grey slaty sandstones, in the counties of Banff, Nairn, Moray, Cromarty, Caithness, and in Orkney, rich in ichthyolites of peculiar forms, belonging to the genera _Pterichthys_ (fig. 400.), _Coccosteus_, _Diplopterus_, _Dipterus_, _Cheiracanthus_, and others of Aga.s.siz.

Five species of _Pterichthys_ have been found in this lowest division of the Old Red. The wing-like appendages, whence the genus is named, were first supposed by Mr. Miller to be paddles, like those of the turtle; but Aga.s.siz regards them as weapons of defence, like the occipital spines of the River Bull-head (_Cottus gobio_, Linn.); and considers the tail to have been the only organ of motion. The genera _Dipterus_ and _Diplopterus_ are so named, because their two dorsal fins are so placed as to front the a.n.a.l and ventral fins, so as to appear like two pairs of wings. They have bony enamelled scales.

_South Devon and Cornwall._--A great step was made in the cla.s.sification of the slaty and calciferous strata of South Devon and Cornwall in 1837, when a large portion of the beds, previously referred to the ”transition” or most ancient fossiliferous series, were found to belong in reality to the period of the Old Red Sandstone. For this reform we are indebted to the labours of Professor Sedgwick and Sir R. Murchison, a.s.sisted by a suggestion of Mr. Lonsdale, who, in 1837, after examining the South Devons.h.i.+re fossils, perceived that some of them agreed with those of the Carboniferous group, others with those of the Silurian, while many could not be a.s.signed to either system, the whole taken together exhibiting a peculiar and intermediate character. But these paleontological observations alone would not have enabled us to a.s.sign, with accuracy, the true place in the geological series of these slate-rocks and limestones of South Devon, had not Messrs. Sedgwick and Murchison, in 1836 and 1837, discovered that the culmiferous or anthracitic shales of North Devon belonged to the Coal, and not, as preceding observers had imagined, to the transition period.

As the strata of South Devon here alluded to are far richer in organic remains than the red sandstones of contemporaneous date in Herefords.h.i.+re and Scotland, the new name of the ”Devonian system” was proposed as a subst.i.tute for that of Old Red Sandstone.

The rocks of this group in South Devon consist, in great part, of green chloritic slates, alternating with hard quartzose slates and sandstones.

Here and there calcareous slates are interstratified with blue crystalline limestone, and in some divisions conglomerates, pa.s.sing into red sandstone.

The link supplied by the whole a.s.semblage of imbedded fossils, connecting as it does the paleontology of the Silurian and Carboniferous groups, is one of the highest interest, and equally striking, whether we regard the _genera_ of corals or of sh.e.l.ls. The _species_ are almost all distinct.

Among the more abundant corals, we find the genera _Favosites_ and _Cyathophyllum_, common on the one hand to the Mountain limestone, and on the other to the Silurian system. Some few even of the _species_ are common to the Devonian and Silurian groups, as, for example, _Favosites polymorpha_ (fig. 401.), very abundant in South Devon.

[Ill.u.s.tration: Fig. 401. _Favosites polymorpha_, Goldf., S. Devon.

From a polished specimen.

_a._ portion of the same, magnified to show the pores.]

The _Cyathophyllum caespitosum_ (fig. 402.) and _Porites pyriformis_ (fig.

424. p. 356.) are more peculiarly characteristic of the Devonian rocks.

In regard to the sh.e.l.ls, all the brachiopodous genera, such as _Terebratula_, _Orthis_, _Spirifer_, _Atrypa_, and _Productus_, which are found in the Mountain limestone, occur, together with those of the Silurian system, except the _Pentamerus_. Some forms, however, seem exclusively Devonian, as for example, _Calceola sandalina_ (fig. 403.) and _Strygocephalus Burtini_ (fig. 404.), which have been met with both in the Eifel, in Germany, and in Devons.h.i.+re, in the very lowest Devonian beds.

[Ill.u.s.tration: Fig. 402. Cyathophyllum.

_a._ _Cyathophyllum caespitosum_, Goldf., Plymouth.

_b._ a terminal star.

_c._ vertical section exhibiting transverse plates, and part of another branch.]

Among the peculiar lamellibranchiate bivalves, also common to Devons.h.i.+re and the Eifel, we find _Megalodon cucullatus_ (fig. 405.). Several spiral univalves are abundant, among which are many species of _Pleurotomaria_ and _Euomphalus_. Among the Cephalopoda we find _Bellerophon_ and _Orthoceras_, as in the Silurian and Carboniferous groups, and _Goniat.i.te_ and _Cyrtoceras_, as in the Carboniferous. In some of the upper Devonian beds, a sh.e.l.l, resembling a flattened _Goniat.i.te_, occurs, called _Clymenia_, by Munster (_Endosiphonites_, Ansted.[347-A]).

[Ill.u.s.tration: Fig. 403. _Calceola sandalina_, Lam. Eifel; also South Devon.

_a._ both valves united.

_b._ inner side of opercular valve.]