SILURIAN ANIMALS. 



317 



mouthed or beakless shells, the former being carnivorous and the latter 

 herbivorous, then only the smooth-mouthed or beakless shells have 

 been found in the Silurian. The beaked-shelled 

 are usually regarded as the more highly-organ- 

 ized class. The affinities of Conularia (Fig. 359) 

 are little understood. They are usually placed 

 among Pteropods. 



Fig. 356. 



Fig. 357. 



Fig. 358. 



Fig. 



Figs. 356-359.— Silurian Gasteropods and Pteropods: 356. Cyrtolites compressus (after Hall). 

 357. Cyrtolites Trentonensis (after Hall). 358. Cyrtolites Dyeri (after Meek), 359. Conularia 

 Trentonensis (after Hall), a Pteropod. 



Cephalopods — Chambered Shells. — These are by far the most highly 

 organized of Mollusks, and the most powerful among Invertebrates. 

 They are represented in the present seas c 



by the Nautilus, the Squids, and the Cut- 

 tle-fishes. If we divide all known Cephal- 

 opods, living and fossil, into Dibranchs 

 (two - gilled) and Tetrabranchs (four- 

 gilled), the former being naked and the 

 latter shelled, then, at the present time, 

 the Dibranchs, or naked, vastly predomi- 

 nate, there being only a single genus of 

 shelled or Tetrabranchs known, viz., the 

 Nautilus, and of this genus only three or 

 four species. In the Silurian age, and for 

 many ages afterward, only the shelled existed, 

 are decidedly the higher in organization. 



Again, if we divide chambered shells into those having simple septa 

 and central or subcentral tube or siphuncle (Nautiloid), and those hav- 

 ing septa plaited at their junction with the shell (plaited suture) and 

 dorsal tube (ammonoid), then in the Silurian age the former only were 

 represented. 



Again, if we divide the Nautiloids into straight-shelled and coiled- 

 shelled, then the straight-chambered shells greatly predominated. 

 Straight-chambered shells are called Orthoceratites (opOos, straight ; 

 Kepa?, horn). The Orthoceratites, therefore, are a very striking feat- 

 ure of the Silurian age. They may be defined as straight-chambered 



Pearly Nautilus (Nautilus 

 pompilius): a, mantle; b, its dorsal 

 fold; c, hood; o, eye; I, tentacles; 

 /, funnel. 



The naked or Dibranchs 



