GENERA OF SHELLS. 99 



Sect. 1.— POLYTHALAMOUS CEPHALAPODA. 



Shell multilocular, wholly or partly developed, inserted in the 

 posterior part of the body of the animal, often adhering. 



ORTHOCERATA. 



Shell straight or nearly so ; no spiral. 

 Most of the shells of this family are unknown 

 except in the fossil state. 



Belemnites. Shell straight, elongated, 

 conical, formed of two distinct and separable 

 parts ; viz. external, a solid sheath, filled at the 

 upper part, with a conical cavity ; internal, a 

 conical nucleus, pointed, chambered trans- 

 versely through its whole length, multilocular ; 

 chambers slightly concave on one side, and 

 convex on the other, and perforated by a 

 central siphon. 



The belemnites, which are only found fossil and 

 generally empty, or without the nucleus, are merely 

 the sheath of an elongated-conical mass, not adhe- 

 ing, chambered, and furnished with a siphon like the 

 orthocera and hippurites. 



Orthocera. Shell elongate, straight or 

 slightly arcuated, subconic, striated externally 

 by numerous longitudinal ribs. Chambers 

 formed by transverse partitions perforated by 

 a tube, either central or marginal. 



O. raphanus, fascia, raphanistrum, obliqua, acicula, 

 legumen, 



NoDosARiA. Shell elongate, straight, or 



