Lign. 160. Shells of the family Ammonitidæ.
Chalk and Greensand.
| Fig. | 1.— | Toxoceras Emericianum, and its septum. Hamsey. |
| 2.— | Crioceras Puzosianum. (M. D’Orbigny.) Lewes. | |
| 3.— | Ancyloceras furcatum, and its septum. France. |
In the genus Ancyloceras (incurved horn), [Lign. 160], fig. 3, the whorls are separate, and, at first, spiral (like Crioceras); but afterwards the shell is prolonged, and then inflected at the large extremity, like a Scaphite, but the whorls are not contiguous.
A very large species of Ancyloceras occurs in the Kentish Rag, near Maidstone, some specimens of which are eighteen inches in length. It is figured and described, by the name of Scaphites Hillsii, in the admirable Memoir of Dr. Fitton on the Strata below the Chalk (Geol. Trans. vol. iv. pl. xv.); the present genus was not then established. The Shanklin Sand in the Isle of Wight also contains a gigantic species, which is figured and described by Mr. J. D. Sowerby, in the Geol. Trans., as Scaphites gigas. Ancyloceras occurs also in the Oolite.
TOXOCERAS. HAMITES.
In Toxoceras (bow-horn), [Lign. 160], fig. 1, the shell is slightly curved, like a horn. Two or three species of Toxoceras are found at Hamsey. The tubercles, in the casts, are the bases of spines, with which the back of the shell was armed, as I have ascertained by examples examined in the rock (see Foss. South D. tab. xxiii. fig. 1). The specimens figured of the above two genera occur in the Neocomian strata of France.
Hamites (hook-shaped). [Lign. 161], fig. 1.—Shell involute, spiral, the turns not contiguous; spire irregular, elliptical; the large end reflected towards the spire. The term Hamite, proposed by the late Mr. Parkinson, was formerly given to all the fragments of sub-cylindrical chambered shells, that were bent, or slightly hooked; and the genera Ancyloceras, Toxoceras, &c., have been separated from them, by M. D’Orbigny. But from fossils recently obtained from Cretaceous strata in Pondicherry, and other parts of India, it seems probable that these genera will be found to merge into each other; at present it is convenient to keep up the distinction. The Hamites are distinguished from Ancyloceras, which they most resemble, by their elliptical, irregular spire.
Ptychoceras (folded horn). [Lign. 161], fig. 4.—This is another genus formed from the Hamites. The shell is bent double in the shape of a siphon, and the limbs are united together. The specimen figured is from the Neocomian strata of the Lower Alps.