GEXA. 37 



Gena and other of J. E. Gray's genera purporting to date from 

 "Syn. Brit. Mas. 1840, " were not really described in that publica- 

 tion, which is merely a popular account of the contents of the British 

 Museum. The genera of Mollusks are enumerated on pages 150- 

 156, but without author's names, and without even a species men- 

 tioned as an example ; nor are the new names distinguished in any 

 way from the old. Adams first characterized the genus in 1850 ; 

 and this may be considered the actual date of publication. The 

 genus differs from Stomatella and Stomatia in having no operculum, 

 and in the less spiral shell, with larger body-whorl and finer sculpt- 

 ure. The observations upon the animal show great differences 

 among the few species examined, which will probably require the 

 dismemberment of the genus when more species are known anatomi- 

 cally. The dentition has not been well figured. The animals of G. 

 planulata Lam. and nigra Q. & G. have a large foot, not capable of 

 retraction under the shell ; the muzzle is squarish, as in Trochus ; 

 tentacles subulate, eyes on short peduncles ; there is a pair of 

 arborescent frontal palmettes, and the epipodium bears no lateral 

 filaments. G. Icevis Pse. has a more lengthened form of muzzle ; there 

 are siphons or conduits for the entrance and egress of water, and 

 ciliated filaments on the epipodial line (see description of animal 

 under that species, and pi. 55, fig. 16). To the last-named species 

 Fischer has given the name Plocamotis. 



The shells are separable by slight but apparently constant features 

 into two sections ; but it remains to be seen whether their characters 

 are correlated with those of the soft parts. 



Sub genera. 



GENA, s. s. Shell with the body- whorl all over finely spirally 

 striated. Animal without epipodial cirri or lengthened cervical 

 water conduits. Type, G. planulata Lam. 



PLOCAMOTIS Fischer. Shell with polished body-whorl, the penul- 

 timate whorl and base finely spirally striated. Animal with epipodial 

 cirri and cervical water conduits. Type, G. Icevis Pse. 



NOTE. The measurements of length and breadth are taken 

 parallel to the plane of the peristome. The length is measured along 

 the line a, b in fig. 20, pi. 2 ; and the breadth along the line c, d. 



