284 



MOLLUSCA. 



Orthocerata are pre-eminently fossils of the Silurian, Devonian, 

 and Carboniferous Rocks. They are, however, found in the 

 Permians, and, passing into the Mesozoic series, they make 

 their last appearance near the summit of the Triassic Rocks. 

 They sometimes attained an enormous size, occasionally ex- 

 ceeding six feet in length, with a diameter of more than a foot. 

 Some idea of the vast numbers of these Cephalopods in the 

 Palaeozoic seas may be obtained from the statement that M. 

 Barrande enumerates more than five hundred species as occur- 

 ring in the small Silurian basin of Bohemia alone. The 

 numerous species of Orthoceras are divided by the above- 

 named distinguished palaeontologist into two principal sections 

 the Short-coned Orthoceratites, and the Long-coned Ortho- 

 ceratites according as the shell has the form of a short cone 

 with a large apical angle, or of a prolonged cone with a small 

 apical angle. The first of these groups is a very small one, and 

 almost all the more common forms come into the second group. 

 The nature of the siphuncle is very different in different 

 Orthocerata, and more or less well marked sub-genera have 

 been founded upon the characters of this structure. In the 

 sub-genus Huronia the siphuncle is of very large size, each 

 joint being cylindrical below but inflated above, the outer 

 walls of the siphuncle being connected with an internal central 



tube by radiating plates. In the forms 

 termed Cochleati the siphuncle consists 

 of a succession of spheroidal bead-like 

 joints. In the sub-genus Endoceras the 

 siphuncle is very large, marginal, ex- 

 centric, or central, and it is partitioned 

 off by funnel-shaped diaphragms. There 

 is, however, considerable difference of 

 opinion as to the true nature of the 

 siphuncle in this sub-genus. Lastly, in 

 the sub-genus Gonioceras the transverse 

 section of the shell is flattened, and the 

 sutures are undulated. 



The genus Cyrtoceras (fig. 254) very 

 closely resembles Orthoceras, but the 

 shell is curved instead of being straight, 

 and the siphuncle is either sub-central, 

 or is more commonly internal i.e., on 

 the concave side of the shell. Cyrtoceras has about the same 

 vertical range as Orthoceras, ranging from the Silurian through 

 all the Palaeozoic formations, and disappearing in the Trias. 

 The genus is characteristically Silurian, and M. Barrande has 



Fig. 254.- -Cyrtoceras iso- 

 dorus (Billings). Lower 

 Silurian. 



