174 



acute, slightly sinuated; the posterior largest and rounded. One elongated 

 and arched, transverse muscular impression, situated under the posterior 

 margin. 



P. ungarica, P. intorta, P. subrufa, G fossil species. 



4 mitrula, 



CalyptRjEa. Shell conoidal, summit vertical, imperforate, subacute; base 

 orbicular; the cavity furnished with a convolute lamina, or a spiral diaphragm. 



C. extinctorium, C. equestris, C. anguluta, C. peziza, 



' laevigata, ' tectum-sinense, ' poculum, ' scutellata. 



Crepidula. Shell ovate or oblong, the back almost always convex, con- 

 cave underneath; the spire very much inclined towards the margin; the aper- 

 ture partly closed by a horizontal lamina. 



C. fornicata, C. aculeata, C. dilata, C. Peruviana. 



' porcellana, ' uguiformis, 



Ancylus. Shell thin, obliquely conical, the summit pointed, inclined 

 backwards; aperture oval; the margin very simple. 

 A. lacustris, A. fluviatilis, A. spina rosae. 



BULL^ANA.— Three Genera. 



Ac era. This genus has no shell. 



A. carnosa. 



Bull^a. Shell very thin, partially rolled and spiral on one side; without 

 columella, and without spire; the aperture very large, dilated at the upper 

 part. 



B. aperta. 



Bulla. Shell univalve, ovate globular, convolute, no columella, spire not 

 projecting; aperture the whole length of the shell; external margin sharp. 



B. lignaria, B. naucum, B. aplustre, B. fragilis, 



' ampulla, ' physis, « hydatis, ' solida. 



1 striata, ' fasciata, « cornea, 



