GENERA OF SHELLS. 101 



equidistant, concave without, with a lateral, 

 interrupted siphon. Aperture orbicular. 



S. Peronii, — (Nautilus spirula, Lin.) 



Spirolinites. Shell multilocular, partly 

 twisted into a discoidal spiral ; whorls con- 

 tiguous, the last terminating in a straight line. 

 Septa transverse, perforated by a tube. 



DistingLiisliRd from spirula by the contiguity of the 

 whorls. Only known in the fossil state ; very small 

 shells. 



LiTUOLiTES. Shell multilocular, partly 

 twisted into a discoidal spiral ; whorls con- 

 tiguous, the last terminating in a straight line. 

 Chambers irregular, septa transverse, simple, 

 (no siphon) ; the last septum pierced with 

 from three to six holes. 



Small fossil shells ; the septa which form the cham- 

 bers are at unequal distances, and inclined to one 

 another; some species have scarcely one complete 

 turn of the spiral. 



CRISTATA. 



Shell semi-discoidal ; spire eccentric. 



Renulites. Shell kidney-shaped, flattened, 

 furrowed, multilocular ; chambers linear ; con- 

 tiguous, curved round a marginal axis ; those 

 farthest from the axis the longest. One fossil 

 species. 



Cristellaria. Shell semidiscoid, multilo- 



