VALVATID^. 33 



species was "not characterised:" overlooking the 

 fact that I pointed out the very marked difference 

 between them found in the young shell ; thus firmly 

 establishing the distinction which some concholo- 

 gists had only considered as a sexual peculiarity, 

 the most ventricose being the female. 



Fam. 5. VALVATID^. 



Rostrum annulated, elongate, bifid. Tentacles equal, 

 elongate, tapering, rather blunt ; eyes on a 

 small tubercle on the inner side of their base. 

 Jaws, two, lateral, narrow, approaching each 

 other above, where there is a third rudimentary 

 jaw, like a small tubercle. (^Moquin, 32. t. 

 41. f. 6.) Mantle-edge thin, with a tentacle-like 

 appendix on the right side, which has been 

 called the branchial thread. Vent on the right 

 side, under the mantle. Gills plumose, exserted 

 when the animal is expanded, formed of an 

 oblong, tapering, conical process, furnished on 

 each side with a series of spirally-twisted laminse, 

 placed opposite to each other, (f. 9. 6.) Foot 

 truncated, nicked slightly, and produced at each 

 angle in front, rounded behind. The male 

 organ is like a third tentacle, situated behind 

 the base of the right tentacle ; the orifice of 

 the female organ is under the collar on the 

 right side of the vent. {JSIoquin, t. 41. f. 5.) 

 " Hermaphrodite ; " oviparous ; eggs emitted 

 singly ; fluviatile, eating aquatic plants ; rarely 



D 



