GENERA OF SHELLS. 99 



SECT. I. POLYTHALAMOUS CEPHALAPODA. 



Shell multilocular, wholly or partly developed, inserted in the 

 posterior part of the body of the animal , of ten 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, 



NODOSARTA. Shell elongate, straight, or 



