104 GENERA OF SHELLS. 



that he possesses some species which are found in the 

 sea near TenerifFe, at the depth of 125 feet. 



Placentula. Shell orbicular, convex above 

 and below, multilocular. Aperture oblong, 

 narrow, disposed as a ray in the inferior disc, 

 or on both discs. 



P. pulvinata, asterisans. 



NAUTILACEA. 



Shell discoidal, spire central, chambers short, not extending 

 from the centre to the circumference. 

 The nautilacea differ from the radiolata, in having 

 the spire composed of several whorls, wherefore the 

 chambers cannot extend from the centre to the cir- 

 cumference ; their spire is also complete, which that of 

 the radiolata never is. 



DiscoRBiTEs. Shell discoidal, spiral, mul- 

 tilocular ; sides simple. All the whorls visible, 

 naked, contiguous to one another. Septa 

 transverse, frequent, imperforate. 



The discorbites differs from the nautilus, by having 

 all the whorls of the spire visible and no siphon ; from 

 rotalites, by the aperture not inclining downward to- 

 wards the base, and the spire not rising into a cone. 



SiDEROLiTES. Shell multilocular, discoidal ; 

 whorls contiguous, not visible externally ; disc 

 convex on both sides, and loaded with tuber- 

 cular points ; circumference bordered with 

 unequal, radiating lobes. Septa transverse, 

 imperforate. Aperture distinct, sublateral. 



