Lamarck's Genera of Shells. 243 



vide the chambers, radiate from the centre or axis of the shell 

 towards the circumference, so that the chambers are slightly 

 conical. 



One species. Rotalites trochidiformis*. 



Shell conoidal ; whorls carinate ; lower side granular. Fossil, 

 Grignon. PI. vi. Fig. 222. 



Lenticulitest. 



Shell sublenticular, 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, pro- 

 duced on both sides like radii. Aperture narrow, projecting over 

 the penultimate whorl. 



The lenticulites are distinguished from the rotalites and dis- 

 corbites, by the lateral prolongation of the chambers and septa, 

 and from the nautilus, by not having the siphon of that shell. 

 They are very similar in structure to the nummulites, but they 

 differ from them by the prolongation of the chambers, fyc, and by 

 the projection of the aperture over the penultimate whorl. They 

 are chiefly found in the fossil state, but Lamarck tells us that he 

 possesses some recent species of this genius, which were found in 

 the 3ea near TenerifFe, at the depth of 125 feet. He describes only 

 three fossil species, but adds in a note, that the nautilus calcar., and 

 nautilus crispus, of Gmelin, as well as the nautUus calcar of Fichtel, 

 appear to be distinct species of lenticulinae, and must be added to 

 those he has described. 



Type. Lenticulites rotulala J. 



Shell orbicular ; margin acute ; discs somewhat gibbous on both 

 sides. Fossil, Meudon. PI. vi. Fig. 223. 

 3. Placentula§. 



Shell orbicular, discoidal, convex above and below, multilocular ; 

 aperture oblong, narrow, lying like the radius of a circle on the 

 lower disc, or on both discs. 



The placentulae are divided internally into several chambers, 



* Trochus-shapcd. f Lenticula, a little lentil. 



I From rotula, a little wheel. § A little cake. 



R 2 



