228 



variation that I found in the specimen sent to me 

 under the two names, induce me for the present to 

 consider them as only variations of the same, pro- 

 bably produced by some local situation, as the dif- 

 ference in the depth of the water, or its being still or 

 running. Mr. Hincks informs me, the first variety 

 is always found in very small plashes of water, or in 

 water among grass, while the larger one is found in 

 canals and nearly still rivers, which may account for 

 all the difference between them ; for we have often 

 been inclined to consider varieties as distinct, because 

 they were found in different situations, whereas the 

 difference of situation may be the only cause of the 

 variation, which would probably disappear if they 

 were placed and allowed to breed in similar circum- 

 stances to the other variety. 



Mr. Jenyns informs me, that he has a British 

 specimen which he thought was the Physa alba of 

 Turton (which is undoubtedly a Sicilian species), 

 differing from the general form of Physa fontinalis 

 in the hinder part of the last whorl being rather 

 angular, as in Limnceus stagnalis. 



The eggs are similar to those of Planorbis and 

 Limnceus, in groups forming oblong or rather de- 

 pressed globular masses attached to the surface of 

 leaves. (See Pfeiffer, t. 8. f. 18. ; see also Jeffreys, 

 Linn. Trans, xvi. 380.) 



7. APLEXUS Fleming. (Aplexus.) 



Animal exactly like Physa ; but the mantle edge is 

 simple and not reflected over the shell, and the 

 tentacles without any auricle at the base, the 



