228 
LIMNJEAD^. 
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 he 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. 1—8,; sqq dim 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 
