20 MOLLUSCA. CEPHALOPODA. 



Gen. 1 4. POLYSTOMKLLA, Lam. 



Shell disciform, multilocular, with contiguous turns, not visible 

 externally, and marked with furrows traversing the whorls ; 

 aperture composed of several holes variously disposed. ( Fossil.) 

 Gen. 15. SIDEROLITES, Lam. 



Shell multilocular, disciform, with contiguous turns, not con- 

 spicuous externally ; the disk convex on either side, and 

 covered with tubercular dots; the circumference margined 

 with unequal radiating lobes ; septa transverse, imperforate ; 

 aperture distinct, somewhat lateral. (Fossil.) 



Gen. 16. DISCORBIS, Lam. 



Shell disciform, spiral, multilocular, with simple walls ; all the 

 turns visible, exposed, and contiguous; transverse septa 

 numerous, imperforate. (Fossil.) 



FAMILY III. RADIOLACEA. 



Shell disciform, with a central spire, and elongated radiating 

 cells, which extend from the centre to the circumference. 



Gen. 17. PLACENTULA. 



Shell orbicular, convex above and beneath, multilocular ; aper- 

 ture oblong, narrow, disposed like a radius in the inferior disk, 

 or in both. (Fossil.) 



Gen. 18. LENTICULINA, Lam. 



Shell sublenticular, spiral, multilocular ; the outer margin of 

 the turns complicate, and extending above and below to the 

 centre ; septa imperforate, curved, prolonged on both sides in 

 the form of radii ; aperture narrow, projecting over the pe- 

 nultimate turn. (Fossil.) 



Gen. 19. ROTALIA, Lam. 



Shell orbicular, spiral, convex or conical above ; flat, radiated, 

 and tubercular beneath, multilocular ; aperture marginal, tri- 

 gonal, resupinate. (Fossil.) 



FAMILY IV. SPHERULACEA. 



Shell globular, spheroidal or oval, with whorls mutually enve- 

 loping, or with cells contiguous and forming a tunic. 



The shells of this family are small, multilocular, spheroid or oval ; some without 

 any other cavity than that of their cells, and with the whorls mutually enveloping 

 one another ; while others are furnished with a particular interior cavity, formed of 

 a train of elongated narrow and contiguous cavities, forming by their union a cover- 

 ing, which surrounds the central cavity. 



Gen. 20. MELONIA, Lam. 



Shell subspherical, multilocular, with a central spire ; whorls 

 contiguous, enveloping, tuniciform ; cells narrow, numerous ; 

 septa imperforate. (Fossil.) 



