CEPHALOPODA 83 



whirls quite disunited constitute the genus Trigonoceras, M'C. 

 (= Nautiloceras, d'Orb.) The Silurian genus Trochoceras, Ban, 

 is a spiral Nautilus. Clymenia, a characteristic Devonian 

 fossil, has angular sutures and an internal siphuncle ; it may 

 perhaps be coiled up ventrally like the Spirilla. The tertiary 

 shell called Nautilus zic-zac (Aturia, Br., fig. 19, i, 2), which 

 is so widely distributed in Europe, America, and India, has a 

 siphuncle nearly marginal when young, but gradually becom- 

 ing more central in the adult : it has no special relation to 

 Clymenia. 



Those species of Orthocerata in which the aperture is con- 

 tracted form the genus Ajnoceras, Fischer ( = Poterioceras, 

 M'C), or when also curved, the Oncoccras of Hall. In Bar- 

 rande's genus Ascoceras (fig. 20, 9), the shell is flask-shaped, 

 the chambered and siphunculated apex being apparently 

 deciduous ; the aperture is contracted, and the air-chambers 

 occupy only the dorsal half of the shell. In Phragmoceras 

 (fig. 20, 7), the shell is slightly curved to the ventral side, and 

 the aperture is remarkably contracted, the opening for the 

 respiratory funnel being nearly distinct from the cephalic 

 aperture. In Cyrtoceras the curvature is dorsal. 



In some other members of this family the siphuncle attains 

 a remarkable size or extraordinary complexity. 



In Camaroceras (fig. 20, 4), the siphuncle is lateral, quite 

 simple, and equal to half the diameter of the shell. Casts of 

 these great siphuncles were called " Hyolites " by Eichwald ; 

 they frequently contain small shells of Orthoceras, Bellerophon, 

 and other genera. In some species the siphuncle is strength- 

 ened internally by repeated layers of shell, or partitioned off 

 by a succession of funnel-shaped diaphragms ; these constitute 

 the genus Endoceras of Hall. The same author has given the 

 name Discosorus to a fossil which is evidently the siphuncle of 

 some very delicate and perishable chambered shell (fig. 20, 6). 

 In those Orthocerata with siphuncles most nearly resembling 



