Part 46 (1/2)

Magnesian limestone, Humbleton Hill, near Sunderland.[303-A]]

Sh.e.l.ls of the genera _Spirifer_ and _Productus_, which do not occur in strata newer than the Permian, are abundant in this division of the series in the ordinary yellow magnesian limestone. (See figs. 337, 338.)

[Ill.u.s.tration: Fig. 337. _Productus calvus_, Sow. Min. Con. Syn. _Productus horridus_, Bronn's Index, &c., King's Monogr., &c.; _Leptaena_, Dalman.

Magnesian Limestone.]

[Ill.u.s.tration: Fig. 338. _Spirifer undulatus_, Sow. Min. Con. Syn.

_Triogonotreta undulata_, King's Monogr.

Magnesian Limestone.]

_The compact limestone_ (No. 4.) also contains organic remains, especially corallines, and is intimately connected with the preceding.

Beneath it lies the _marl-slate_ (No. 5.), which consists of hard, calcareous shales, marl-slate, and thin-bedded limestones. At East Thickley, in Durham, where it is thirty feet thick, this slate has yielded many fine specimens of fossil fish of the genera _Palaeoniscus_, _Pygopterus_, _Coelacanthus_, and _Platysomus_, genera which are all found in the coal-measures of the carboniferous epoch, and which therefore, says Mr. King, probably lived at no great distance from the sh.o.r.e. But the Permian species are peculiar, and, for the most part, identical with those found in the marl-slate or copper-slate of Thuringia.

[Ill.u.s.tration: Fig. 339. Restored outline of a fish of the genus _Palaeoniscus_, Aga.s.s. _Palaeothrissum_, Blainville.]

The _Palaeoniscus_ above mentioned belongs to that division of fishes which M. Aga.s.siz has called ”Heterocercal,” which have their tails unequally bilobate, like the recent shark and sturgeon, and the vertebral column running along the upper caudal lobe. (See fig. 340.) The ”h.o.m.ocercal” fish, which comprise almost all the 8000 species at present known in the living creation, have the tail-fin either single or equally divided; and the vertebral column stops short, and is not prolonged into either lobe. (See fig. 341.)

[Ill.u.s.tration: Fig. 340. Shark.

_Heterocercal._]

[Ill.u.s.tration: Fig. 341. Shad. (_Clupea_, Herring tribe.)

_h.o.m.ocercal._]

Now it is a singular fact, first pointed out by Aga.s.siz, that the heterocercal form, which is confined to a small number of genera in the existing creation, is universal in the Magnesian limestone, and all the more ancient formations. It characterizes the earlier periods of the earth's history, when the organization of fishes made a greater approach to that of saurian reptiles than at later epochs. In all the strata above the Magnesian limestone the h.o.m.ocercal tail predominates.

A full description has been given by Sir Philip Egerton of the species of fish characteristic of the marl-slate in Mr. King's monograph before referred to, where figures of the ichthyolites which are very entire and well preserved, will be found. Even a single scale is usually so characteristically marked as to indicate the genus, and sometimes even the particular species. They are often scattered through the beds singly, and maybe useful to a geologist in determining the age of the rock.

[Ill.u.s.tration: Fig. 342. _Palaeoniscus comtus_, Aga.s.siz. Scale magnified. Marl-slate.]

[Ill.u.s.tration: Fig. 343. _Palaeoniscus elegans_, Sedg. Under surface of scale magnified. Marl-slate.]

[Ill.u.s.tration: Fig. 344. _Palaeoniscus glaphyrus_, Ag. Under surface of scale magnified. Marl-slate.]

[Ill.u.s.tration: Fig. 345. _Coelacanthus caudalis_, Egerton. Scale showing granulated surface magnified. Marl-slate.]

[2 Ill.u.s.trations: Scales of fish. Magnesian limestone.

Fig. 346. _Pygopterus mandibularis_, Ag. Marl-slate.

_a._ Outside of scale magnified.

_b._ Under surface of same.

Fig. 347. _Acrolepis Sedgwickii_, Ag. Marl-slate.]