18 HELIX, 



into the cavernous umbilicus of their shells ; this umbilicus is 

 constricted with the formation of the last two whorls, and the 

 eggs and young are sometimes still more effectually retained by 

 a temporary shelly plate which partially covers the opening. 



Section TROCHOMORPHA, Albers, 1850. 



Subperforated to umbilicated, trochiform, with more or less 

 obtuse apex ; whorls somewhat flattened, the last with a carinated 

 periphery, and planulate base ; aperture depressed, oblique, peris- 

 tome simple. 



A large group, the metropolis of which is central Polynesia, 

 but some of the species are East Indian. Trochomorphoides 

 and Ganesella, Nevill, are scarcely distinguishable. 



Liocystis, Morch, contains a species having a sigmoid colu- 

 mella. I have followed Pfeiffer in including it here. 



Subsection NIGRITELLA, Albers, 1860. 



Includes the typical forms of TrochomorpJia, as distinguished 

 from the following subsections. 



Subsection VIDENA, H. and A. Adams, 1852. 



Widely umbilicated, thin, discoidal, carinated at the periphery, 

 flat or somewhat elevated above, flatly convex below ; aperture 

 widely subtriangular, the peristorne simple, sharp or slightly 

 thickened. Mostly Polynesian and Australasian. H. ACUTI- 

 MARGO, Pfr. Philippines. 



Subsection THYSANOTA, Albers, I860. 



Narrowly umbilicated, trochiform, thin; whorls scarcely con- 

 vex, base carinated, with rigid hairs ; aperture angularly lunar, 

 lip simple, acute, columellar margin slightly expanded. H. 

 GUERINI, Pfr. India. 



The New Zealand group Thera, Hutton, appears to be iden- 

 tical. 



Subsection PARARHYTIDA, Ancey. 



Narrowly umbilicated, conical-discoidal, obliquely plicately 

 striate, thin, diaphanous, periphery acutely carinated. H. DIC- 

 TYODES, Pfr. New Caledonia. 



