GENERA OF SHELLS. 103 



veloping. Chambers narrow, elongated and 

 numerous ; septa imperforate. 



RADIOLATA. 



Shell discoidal, spire central ; chambers elongated, radiating 

 from the centre to the circumference. 



From the character of the shells of this family, it 

 follows that their spire can have but one turn, and is 

 consequently false or imperfect. 



ROTALITES. Shell orbicular, spiral, convex 

 or conoidal at the upper part ; flattened, 

 radiating, and tubercular at the lower ; multi- 

 locular. Radii wavy ; aperture marginal, tri- 

 angular, inclined towards the base. 



The rotalites are very small shells, widest at' the base, 

 with the whorls contiguous and distinct. 



LENTICULITES. Shell sub-lenticular, spiral, 

 multilocular ; exterior margin of the whorls 

 triplicate, extending over the interior whorls, 

 both above and below, to the centre of the 

 shell. Septa entire, curved, produced on both 

 sides like radii. Aperture narrow, projecting 

 over the penultimate whorl. 



The lenticulites are distinguished from the rotal- 

 ites and discorbites, by the lateral prolongation of the 

 chambers and septa, and from the nautilus by not 

 having the siphon. They are similar to nummulites, 

 but differ by the prolongation of the chambers and by 

 the projection of the aperture over the penultimate 

 whorl. They are chiefly fossil, but Lamarck tells us 



