NAUTILID.E. 



69 



discoidal ; bod7-cliaml3er small ; aperture of the shell small, 

 sometimes extremely contracted; siphuncle complicated. The 

 Orthoceratidce commence in the lowest Silurian deposits, and 

 attain their maximum of development in the Silurian rocks. 

 The family is well represented in the Devonian and Carbon- 

 iferous rocks, but is much reduced in numbers in the Per- 

 mians. The last appearance of the family is in the Triassic 

 rocks, where it is represented by the genera OrtJwccras and 

 Cyrtoceras. The chief genera of this sub-family are Ortho- 

 ceras, Gomplioceras, Phragmoceras, Cyrtoceras, and Ascoceras. 

 In the genus OrtJwceras (fig. 455) the shell is straight, the 



Fig. 455. — Fragment of Orthoceras (Ormo- 

 ceras) crebriseptum — Cincinnati Group, 

 North America, of the natural size. The 

 lower figure is a section showing the air- 

 chambers, and the form and position of 

 the siphuncle. (After Billings.) 



Fig. 456.— Restoration of Orthoceras, 

 the sheU being supposed to be divided 

 vertically, and only its upper part being 

 shown. a, Arms ; /, Muscular tube 

 ("funnel") by which water is expelled 

 from the mantle-chamber ; c, Air-cham- 

 bers ; s, Siphuncle. 



siphuncle central or excentric, often of a very complex struc- 

 ture, and the aperture of the shell sometimes contracted. 

 The Orthocerata are pre-emmently fossils of the Silurian, 



