VALVATID^E. 211 



as also in the variety pellucida ; similis is 0-15 

 long, and o-i broad. . Ventrosa is o f 2 long, and 0*125 

 broad. This little shell is abundant, and has been 

 called by several names. It is said to be the Cyclostoma 

 acutum of Draparnaud. It chiefly differs from H. 

 similis in having the spire long, the suture not chan- 

 nelled, and on the umbilical clinch being much 

 smaller. The varieties of H. ventrosa are minor, of 

 which the spire is small ; decollata, of which the shell 

 is eroded, and the spire truncate ; ovata, having only 

 four whorls, and a short spire ; elongata, having a 

 long spire, and sometimes 8 whorls ; pellucida has a 

 white, transparent shell. It is found throughout 

 England and Wales, and even in Ireland. 



The genus Valvata are very minute animals. 

 The body of V. piscinalis is clear yellowish-gray, 

 with small white spots; the shell is of a depressed 

 conical form, opaque, brownish-yellow, with 6 whorls, 

 the last being nearly, half the shell. The varieties 

 are depressa, with shell more depressed; subcylin- 

 drica, with the spire more produced, and flattened 

 at the top ; acuminata, with a sharp-pointed spire. The 

 usual length of piscinalis is 0*25 ; the breadth 0*275. 

 In V. cristata the body is dark, with black spots. 

 The shell forms a flat coil, concave, rather solid 

 beneath, semi-transparent yellowish, or gray horn- 

 colour, very thin epidermis, 5 whorls, the last greater 

 than all the others. The operculum is formed like an 

 inverted potlid. These species inhabit lakes, canals, 

 and ditches throughout Britain. 



Those species of the order Pulmonolranchiata, or 

 with a lung-like gill, which have no operculum have 

 p 2 



