100 GENERA OF SHELLS. 



slightly arcuated, subconic, with smooth nodes. 

 Chambers formed by transverse partitions^ 

 perforated either at the centre or near the 

 margin. 



N. radicula, dentalina,'"siphunculus. 



HiPPURiTEs. Shell tubular, cylindrico-coni- 

 cal, straight or shghtly curved, thick, multilo- 

 cular ; septa transverse. An internal, lateral 

 canal, formed by two longitudinal, obtuse, 

 converging edges. The last chamber closed 

 by a thick, solid operculum ; edges of the 

 operculum bevelled, and accurately adjusted 

 to the orifice of the chamber. Fossil. 



CoNOLiTEs. Shell conical, straight, slightly 

 bent ; sheath thin, distinct from the contained 

 nucleus. Nucleus sub-separable, multilocular, 

 divided by transverse septa. 



The cooolites appears to differ from the helemnites, 

 principally in not having the upper portion of the 

 sheath, or external shell, elongated and solid, (in con- 

 sequence of the termination of the cavity for the nu- 

 cleus before it reaches the summit) as in those shells. 



LITUOLATA. 



Shell jidrtly spiral ; the last whorl straight. 



Spirula. Shell cylindrical, thin, nearly 

 transparent, multilocular; partially turned into 

 a discoid spiral ; whorls distant ; the last pro- 

 duced into a right line. Partitions transverse, 



