1 6 NORTH AMERICAN INDEX FOSSILS. 



I. S. fissurella (Hall). (Fig. 1229.) Devonic. 



JNIinute. Sharply depressed central fracture line present in all 

 the compressed specimens. 



Especially abundant in the lower Genesee, where these shells 

 make up the StyHoHna limestone; also other Upper Devonic beds 



:u 



Fig. 1228. Styliola recta ; fy iooi; FiG. 1229. Styliolina fissurella, with 



i/z, shell. Recent, enlarged. (After enlargement. (After Hall.) 

 Adams.) 



of New York, Michigan, Ohio, Indiana, Maryland, Virginia, etc. 

 Also abundant in the Marcellus shale of New York. 



Class CEPHALOPODA Cuvier. 

 The cephalopods are the most highly developed of molluscs, 

 possessing a distinct, well-defined head, a circle of eight or more 

 arms or tentacles which surround the mouth; a funnel-like hypo- 

 nome through which the animal is enabled to eject a stream of 

 water and so propel itself backwards, and a highly developed ner- 

 vous system. The majority of modern cephalopods are naked, or 

 have only a rudimentary internal shell {Dibranchiata) , though one 

 of them, the female Argonauta, secretes a spiral non-septate shell, 

 which, however, is not the homologue of the typical cephalopod 

 shell. Nautilus, the only living representative of the Tetrabran- 

 chiata, is also the only modern cephalopod with a typical external 

 shell (Fig. 1230), and our knowledge of the soft parts and their 

 relation to the shell is wholly derived from this genus.* As the 



*See Griffin, Lawrence, **The Anatomy of Nautilus ponipHius,^'' Memoirs Nat. 

 Acad. Sciences, Vol. VIII., no. 5, 1900. 



