Part 51 (1/2)

[Ill.u.s.tration: Fig. 388. _Cyathocrinites pla.n.u.s_, Miller.

Mountain limestone.]

[Ill.u.s.tration: Fig. 389. _Spirifer glaber_, Sow. Mountain limestone.]

[Ill.u.s.tration: Fig. 390. _Productus Martini_, Sow. (_P. semireticulatus_, Flem.) Mountain limestone.]

Among the spiral univalve sh.e.l.ls the extinct genus _Euomphalus_ (see fig.

391.) is one of the commonest fossils of the Mountain limestone. In the interior it is often divided into chambers (see fig. 391. _d_); the septa or part.i.tions not being perforated, as in foraminiferous sh.e.l.ls, or in those having siphuncles, like the Nautilus. The animal appears, like the recent _Bulimus decollatus_, to have retreated at different periods of its growth, from the internal cavity previously formed, and to have closed all communication with it by a septum. The number of chambers is irregular, and they are generally wanting in the innermost whorl.

[Ill.u.s.tration: Fig. 391. _Euomphalus pentagulatus_, Min. Con.

Mountain limestone.

_a._ Upper side; _b._ lower, or umbilical side; _c._ view showing mouth which is less pentagonal in older individuals; _d._ view of polished section, showing internal chambers.]

[Ill.u.s.tration: Fig. 392. Portion of _Orthoceras laterale_, Phillips. Mountain limestone.]

There are also many univalve and bivalve sh.e.l.ls of existing genera in the Mountain limestone, such as _Turritella_, _Buccinum_, _Patella_, _Isocardia_, _Nucula_, and _Pecten_.[341-A] But the _Cephalopoda_ depart, in general, more widely from living forms, some being generically distinct from all those found in strata newer than the coal. In this number may be mentioned _Orthoceras_, a siphuncled and chambered sh.e.l.l, like a _Nautilus_ uncoiled and straightened. Some species of this genus are several feet long (fig. 392.). The _Goniat.i.te_ is another genus, nearly allied to the _Ammonite_, from which it differs in having the lobes of the septa free from lateral denticulations, or crenatures; so that the outline of these is continuous and uninterrupted (see _a_, fig. 393.). Their siphon is small, and in the form of the striae of growth they resemble _Nautili_. Another extinct generic form of Cephalopod, abounding in the Mountain limestone, and not found in strata of later date, is the _Bellerophon_ (fig. 394.), of which the sh.e.l.l, like the living Argonaut, was without chambers.

[Ill.u.s.tration: Fig. 393. _Goniat.i.tes evolutus_, Phillips.[342-A]

Mountain limestone.]

[Ill.u.s.tration: Fig. 394. _Bellerophon costatus_, Sow.[342-B]

Mountain limestone.]

FOOTNOTES:

[329-A] H. D. Rogers, Trans. a.s.soc. Amer. Geol., 1840-42, p. 440.

[333-A] Trans. of a.s.s. of Amer. Geol., p. 470.

[334-A] Lyell's Second Visit to the U. S., vol. ii. p. 245. American Journ. of Sci., 2d series, vol. v. p. 17.

[335-A] Principles of Geol., p. 696.

[335-B] For changes in climate, see Principles of Geol., chaps.

vii. and viii.

[335-C] Geol. Trans., 2d series, vol. vi. p. 330.

[336-A] Aga.s.siz, Poiss. Foss., lib. 4. p. 62. and liv. 5. p. 88.

[337-A] Goldfuss, Neue Jenaische Lit. Zeit., 1848; and Von Meyer, Quart.

Geol. Journ., vol. iv. p. 51., memoirs.

[338-A] See Lyell's Second Visit, &c., vol. ii. p. 305.