70 CEPHALOPODA. 



Devonian, and Carboniferous rocks. Tliey are, liowever, 

 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 exceeding six feet in length, with a diam- 

 eter of more than a foot. Some idea of the vast numbers 

 of these Cephalopods in the Palseozoic seas may be obtained 

 from the statement that M. Barrande enumerates more than 

 five hundred species as occurring in the small Silurian basin 

 of Bohemia alone. The numerous species of Ortlioceras are 

 divided by the above-named distinguished pakeontologist into 

 two principal sections — the Short-coned Orthoceratites, and 

 the Long-coned Orthoceratites — 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 

 Ortlioccrata, 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 cen- 

 tral tube by radiating plates. In the forms termed Cochkati 

 the siphuncle consists of a succession of spheroidal bead-like 

 joints. In the sub-genus Endoccras the siphuncle is very 

 large, marginal, excentric, or central, and it is partitioned off 

 by funnel-shaped diaphragms. There is, however, consider- 

 able difference of opinion as to the true nature of the si- 

 phuncle in this sub-genus. In Tretoceras there is a nearly 

 central siphuncle, but the uppermost septa are traversed by 

 a deep lateral cavity, wdiich communicates above with the 

 body-chamber. The genus is Lower Silurian. Lastly, in 

 the sub-genus Gonioceras the transverse section of the shell 

 is flattened, and the sutures are undulated. 



The genus Cyrtoceras (fig. 457) very closely resembles 

 Ortlioceras, but the shell is curved instead of being straight, 

 and the siphuncle is eitlier sub-central, or is more commonly 

 internal — i.e., on the concave side of the shell. Cyrtoceras 



