340 
ELEMENTS OE GEOLOGY. 
ncea being an extinct genus of univalve shells (Fig. 325) much 
resembling the Cerithium in external form. The annexed sec¬ 
tion shows the curious and continuous ridges on the columella 
and whorls. 
Oxford Clay.—The coralline limestone, or coral rag,” 
above described, and the accompanying sandy beds, called 
“ calcareous grits,” of the Middle Oolite, rest on a thick bed 
of clay, called the Oxford Clay,” sometimes not less than 600 
feet thick. In this there are no corals, but great abundance of 
cephalopoda, of the genera Ammonite and Belemnite (Figs. 
326 and 327). In some of the finely laminated clays ammon- 
Fig. 326, Fig. 32T, 
Belemnites hastatus* 
Oxford Clay. 
Ammonites Jason, Reinecke. (Syn. A. Mizahethm, Pratt.) 
Oxford Clay, Christian Malford, Wiltshire. 
ites are very perfect, although somewhat compressed, and are 
frequently found with the lateral lobe extended on each side 
of the opening of the mouth into a horn-like projection (Fig. 
327). These were discovered in the cuttings of the Great 
Western Railway, near Chippenham, in 1841, and have been 
described by Mr. Pratt {An. Wat. Ilist..^ ISTov., 1841). 
Similar elongated processes have been also ooserved to 
extend from the shells of some belemnites discovered by Dr. 
Mantell in the same clay (see Fig. 328), who, by the aid of 
this and other specimens, has been able to throw much light 
on the structure of singular extinct forms of cuttle-fish.* 
* See Phil. Trans. 1850, p. 363 • also Huxley, Memoirs of Geol. Survey, 
1864 ; Phillips, Palseont. Soc. 
