244 OVULID^E. 



myself (Am. Jour. Conch., i, 95), and afterwards confirmed by 

 the late Dr. J. Gwyn Jeffreys and others, the animal, under 

 favorable circumstances, sometimes completes its shell by the 

 addition of a thickened lip, when the distinction from the typical 

 group disappears. Ordinarily Simnia has the sharp lip and it 

 may be considered a case of arrested growth, paralleled by that 

 of Planorbis trivolms which only in northern waters takes on 

 its expanded lip along with more vigorous growth and larger size. 



Subgenus CYPHOMA, Bolten, 1798. 



Back of shell with a median transverse obtuse ridge, lip 

 smoothish or feebly crenulated. 0. gibbosa, Linn. 



Ultimus, Montfort, and Carinea, Swainson, are synonyms. 



Subgenus VOLVA, Bolten, 1798. 



Shell ventricose in the middle, both extremities prolonged into 

 canals, aperture narrow, lip inflected and margined exteriorly, 

 without teeth, no columellar plication. 



Mantle-margin with glandular tubercles, foot narrow, folded 

 lengthwise on itself, adapted for creeping on the narrow, rounded 

 branches of gorgonise and corals on which it is supposed to feed. 



Birostra, Swains., and Radius, Montf., are synonyms. 



Subgenus NEOSIMNIA, Fischer, 1884. 



Shell resembling in form the preceding group, but the 

 extremities less prolonged, the columella furnished behind with 

 an oblique callous plication. The habits of the animal appear to 

 be the same as Yolva. 



? Subgenus CRITHE, Gould, 1860. 



Shell ovuliform, rostrated at the extremities, aperture narrow, 

 arcuated ; outer lip simple, base with transverse ridges passing 

 into the aperture and making teeth on the columellar lip. Animal 

 with a long foot, truncate anteriorly, obtuse posteriorly, .tentacles 

 broad, short, triangular, with minute eyes at their exterior base. 

 Described from a single minute species obtained by Stimpson 

 in the China Sea. It is probably a juvenile, and must remain 

 a doubtful group until rediscovered the types having been 

 destroyed. 



