LOWER SILURIAN. 



201 



the partitions is figured separately, so as to show the position and 

 size of the siphuncle. Fig. 355 is a transverse section of another 



Gasteropods. — Fig. 344, Pleurotomaria lenticularis ; 345, Murchisonia bicincta; 346, M. belli- 

 cincta ; 347, Helicotoma planulata ; 348, 349, Belleropkoti bilobatus ; 350, Cyrtolites com- 

 pressus ; 351, C. (?) Trentonensis ; 352, id., dorsal view. 



species, in which the siphuncle is very large. These Orthocerata 

 occupied the place of Fishes in the seas; yet, with their lonaj unwieldy 



Figs. 353-358. 



356, 



Cephalopods. — Pig. 353, a, Orthoceras junceum ; 354. 0. vertebrate ; 355, Ormoceras tenuifilum ; 

 356, a, Cyrtoceras annulatum ; 357, Cryptoceras undatum ; 358, Trocholites Ammonius. 



shells, they must have been sluggish animals. Other related Cephal- 

 opods had the shells coiled (Figs. 357, 358), a much more convenient 



