GENERA OF SHELLS. 105 



PoLYSTOMELLA. Shell discoidal, multilocu- 

 lar; whorls contiguous, not apparent exter- 

 nally, radiated exteriorly by striae or costse 

 running in the direction of the whorls. Aper- 

 ture of many holes variously disposed. 



These characters are common also to the lenticulites, 

 but the aperture of the latter is shnple, whilst that of 

 the polystomella is composed of several holes, differ- 

 ently disposed in different species. 



Pi crispa, costata, planulata, ambigua, 



VoRTiciALis. Shell discoidal, spiral, multi- 

 locular ; whorls contiguous, not apparent ex- 

 ternally ; septa transverse, imperforate, not 

 extending from the centre to the periphery. 

 Aperture marginal. 



The vorticialis differs from nummulites chiefly by 

 having a distinct aperture, and from discorbites, by the 

 spiral whorls not being visible externally. Their axis 

 is central and confounded with the summit of the 

 spire. 



V. craticulata, strigilata, marginata. 



Nummulites. Shell lenticular, attenuated 

 towards the edges ; spire internal, discoidal, 

 multilocular, covered with several thin plates ; 

 exterior side of the whorls triplicate, extending 

 from both sides to the centre of the shell and 

 uniting. Chambers very numerous, small, al- 

 ternate ; septa imperforate, transverse. 



