214 



PALAEOZOIC TIME — LOWER SILURIAN. 



feet long : the genus Ormoceras is peculiar in the beaded form of the siphuncle. 

 Other common species of the Orthoceras family are the Endoceras proteiforme H., 

 and the Gonioceras anceps H. The Endoceras is the most gigantic known, 



Figs 313-318. 





Cephalopods. — Fig. 313, a, Orthoceras junceum; 314, 0. vertebrale; 315, Ormoceras te- 

 nuinlum; 316, a, Cyrtoceras annulatum; 317, Cryptoceras undatum; 318, Trocholites 

 Ammonius. 



having attained a length in some cases of fifteen feet, and a diameter of nearly 

 one foot. In the genus Endoceras (from the Greek Ktpaq, horn, and evSov, within) 

 there is a concentric structure of cone within cone. In the Gonioceras the septa 

 or partitions are very much crowded and have a double curvature, and the 

 siphuncle is central. 



(/.) There are also curved species. Fig. 316 is Cyrtoceras annulatum H. ; 

 a, a transverse section, showing the position of the siphuncle ; fig. 317, Cryp- 

 toceras undatum (Lituites undata H.), an abundant species in the Black 

 River limestone; fig. 318, Trocholites Ammonius of the Trenton; 318 a, a 

 transverse section. In Cryptoceras the spire is open at the outer extremity and 

 the siphuncle is dorsal ; while in Trocholites it is closed and very tightly coiled 

 throughout. Lituites, another genus of the period, which first appeared in the 

 Calciferous, differs from Cryptoceras in having the siphuncle subcentral. The 

 genus Phragmoceras is peculiar in having the mouth of the shell very much con- 

 tracted by a bending inward of the sides. A species, P. immaturum B., occurs 

 in the Black River limestone of Canada. 



