72 



CEPHALOPODA. 



cases the ventral side of the shell is relatively the most 

 convex, but the reverse of this sometimes occurs. The 

 species of Gom^ohoccras range from the Silurian to the Car- 

 boniferous, but belong mainly to the former. M. Barrande 

 enumerates no less than seventy-three species as occurring in 

 the Silurian rocks of Bohemia. 



The genus Ascoccras (fig. 459) comprises some singular 

 forms in which the shell is globular or flask-shaped, and the 



septa do not run at right 



; '\ angles to the axis of the shell, 



; \ but nearly parallel with it, 



\ being at the same time curved 

 in an extraordinary manner, 

 so that the body-chamber is 

 prolonged downwards on one 

 side almost to the bottom of 

 the shell. The air-chambers 

 also are restricted to a portion 

 only of the shell. In Aphrag- 

 mitcs, again, the air-chambers 

 are not persistent. Both these 

 genera are exclusively con- 

 fined to the Silurian rocks, 

 abounding chiefly in the iip- 

 per division of the series. 



Numerous other generic and 

 sub-generic types are included 

 under the Ortlioceratidce, of 

 which the following deserve a 

 passing mention. In the Silurian genus Cyrtoccrma (fig. 

 460, c) the shell is curved, and has the general shape of 

 Cijrtoceras, but it is much more broadly conical, and the si- 

 phuncle is of large size and placed on the dorsal side. Notli- 

 oceras, also Silurian, has a nautiloid shell, with simple septa, 

 and likewise a dorsal siphuncle. Bathinoceras, from the same 

 formation, has the siphuncle composed of a series of funnel- 

 shaped tubes, which fit into one another, their narrow 

 extremities being directed upwards. Lastly, Aulacoceras 

 resembles Orthoceras in general form, but the shell is thick, 



Fig. 459. — Ascocera.s Canadensis (Bil- 

 lings), showing the form of the septa. 

 Lower Siluiian. 



