OF MOLLUSCA. 123 



side ; lateral teeth somewhat similar, laminaceous, toothed on each 

 side ; the apical tooth larger than the rest (fig. 63). 



Valvata piscinalis is androgynous ; the ovules very large at ma- 

 turity. Penis exterior, on the side of the neck like Bithinia, but 

 simple. Female orifice on back of neck, under mantle edge. — Mo- 

 quin-Tandon, Journ. Conch, hi. 246. t. 9. f. 32. 



The Valvatce live in rivers. They are known from the Paludince 

 by the whorls being more circular and not bent in any part by the 

 proximate whorls. The gills are exserted when the animal is ex- 

 panded, and are formed of an elongated tapering conical process, 

 furnished on each side with a series of spirally-twisted laminae placed 

 opposite to each other. On the hinder part of the right side, near 

 the suture of the whorls, is an exserted filiform member like a tenta- 

 culum, but rather shorter and thicker, which is called the branchial 

 thread by Lamarck. 



Valvata tricarinata is found in the small lakes of America, usually 

 under stones, or sheltered by the deserted shell of some freshwater 

 Unio. The shell is usually rendered somewhat opake by an earthy 

 coating, but when removed it has a shining, pearly, emerald-green 

 periostraca. 



Valvata pupoides is very active. The head proboscidiform, half 

 as long as the tentacles, two-lobed in front. Foot tongue-shaped, 

 dilated into two acute angles in front. Gills occasionally protruded 

 to half the length of the right tentacle. Tentacles rather stout. 

 The last whorl of the shell is nearly disjointed. They live in ponds, 

 under stones and submerged sticks. 



Mr. Lea remarks, " I observed a small apple-green globose object 

 (the ova) passing from under the aperture of the shell. This was 

 shortly followed by others, and soon a transparent mass became 

 visible. This mass was passed slowly over the right side of the 

 neck, under the pectiniform moveable branchiae, until entirely dis- 

 charged against the perpendicular sides of the vessel in which it was 

 kept, and there the mass remained attached, the parent having 

 abandoned it immediately." 



III. Opisophthalma. Eyes sessile, on the back, between or 

 rather behind the base of the tentacles. 



A. Operculum spiral. Eyes near the base of the tentacles. Gills 



vascular ? 



Fam. XXIV. ACICULAD^). 



Tentacula subulate. Eyes on the head, rather behind and between 

 the tentacula. Foot oblong. Gills vascular ? Shell spiral, subtur- 

 rited. Operculum horny, spiral, of few rapidly enlarging whorls. 

 Foot divided across (allied to Truncatella and Auricula). 



Terrestrial. 



