126 



Chusan Shells. 

 Drap. 



[No. 2. 

 -H . bi- 



Sub-genus Helisoma, Sw 

 carinata. 



1 . Whorls hardly three. 



2. Shell ventricose. 



3. The spire sunk below the body- 



whorl. 



Sub-genus Plauorbis, 

 JP. corneus. 



1. Spiral whorls few. 



2. Body-whorl ventricose. 



3. (Note. This may be said to be 

 the case in PI. comeus, it mat- 

 ters not which side be regarded 

 as containing the apex). 

 I have not Sowerby's Grenera to refer to for the type of Helisoma, 



but if it be intended for the reception of forms similar to that next 

 to be described, and which is ■ par excellence' of the Heliciform type ; 

 and if Helisoma be an unclassical abbreviation of the hybrid word 

 Helicosonia, in allusion thereto, then the depression of the spire must 

 have reference to the depth and construction of the umbilicus on the 

 really lower side, and the character " shell ventricose" must refer to 

 the convexity (in the more typical species) of the upper side of the 

 shell. If Swainson should not have intended to characterise this 

 type, I would propose the term Helicorbis for it, and would offer the 

 following incomplete sketch of an arrangement. I have omitted 

 Segmentina, Fleming ; as though its singular internal divisions make 

 it represent the Cephalopoda, yet its form does not separate it from 

 the subgenus in which JPlanorbis nitidus will be found, and it is not 

 representative, quoad that structure, of any of the families of the 

 tribe to which it belongs. 



Families of the 

 Phytophaga. 

 1. Helicidce, 



2. Trochidce. 



Analogies. Subgenera of JPlanorhis. 



Typical. Helicorbis? Benson. 



H. hemisphcerula, Benson. 

 H. nitida, Gray's Turton PL 



8, f. 93. 

 H. umbilicalis, Journ. As. 

 Soc. 5. p. 741. 

 Aberrant. H. papyraceus, Benson. 



Subtypical. Trochorbis, Benson. 



Trochorbis trochoides, J. A. 

 S. 5. p. 742. 



