18 RESEARCHES UPON THE HYDROBIIN^ 



tooth being peculiar, as iH all other genera of the Hydrobiinae it 

 is denticulated. This we will consider, for the present, as the 

 dentition of the genus Bytliinella. 



The shells of the Bythinellse differ from those of the Amnicolte 

 in being much more elongated ; they correspond to those of the 

 group Fomaliopsis, as proposed by Tryon, although that name 

 must now have a far diiferent signification on account of the char- 

 acter of its type. The apex of the shell is generally obtuse ; the 

 whirls are tightly coiled, and the* umbilicus generally closed. The 

 tentacles of the animal, according to Moquin-Tandon, are setace- 

 ous and pointed (not obtuse at the extremity, as in Amnicola) ; 

 the verge is bifid, but the longer branch is not coiled about the 

 shorter one ; the foot is narrow, \vith its posterior extremity "con- 

 cealed in great part by the operculum ;'" and the eggs are " globu- 

 lar, hyaline, sometimes arranged symmetrically in two rows, and 

 forming a band fixed to solid bodies.'"^ 



By comparison of these characters with those of Amnicola given 

 above, it will be seen that Bythinella differs from that genus suffi- 

 ciently both in its shell and its soft parts ; in its dentition, and 

 perhaps also in the form of the ova-capsules ; and it therefore 

 should not be united to Amnicola, as proposed by Troschel, Gill, 

 and others. 



The name Hychohia is used for the Bythincllte by Bourguignat^ 

 and some other recent writers, but it would seem preferable to 

 restrict that name to the marine forms, as has been done by 

 Frauenfeld.* The latter author has, however, wrongly used for the 

 Bythinellae^ the name Paludinella, Pfeiffer, which was founded^ 

 on the Truncatella liitorina of Philippi,^ an air-breathing snail 

 having the eyes on the upper sides of the bases of the tentacles, 

 and belonging therefore to a group of gasteropods far difierent 

 from those here treated of. 



' Moquin-Tandon, I. c, p. 517. 



* Moquin-Tandon, /. c, p. 514. Possibly this description of the eggs 

 refers to the true Bythiniae only. 



3 Guerin's Rev. et Mag. de Zool., XIV, 18r,2, 96. 



< Verhandl. der K.-k. Zool.-bot. Gesellschaft in Wien, XIII (1863), 1017. 



6 Loc. cit., XIII, 1863, 199. 



^ Archiv fiir Naturgeschichte, I, 1841, 227. 



' Arch, fiir Naturg., I, 1841, pi. v, fig. 7. See also, on this point, Forbes 

 & Hanley, Brit. Moll., Ill, 133. 



