M1LACOKOA. TEOPIOPODA. LAM1LLIBHANCHIATA. 251 



valves convex, and meeting at a more obtuse angle ; the sur- 

 face shining, the striae faint ; the shell transparent, yellowish- 

 white, and allowing the reddish-white animal to be dimly seen 

 through it. All the individuals, even the young, are oval and 

 convex. The greatest length a twelfth and a-half, the height 

 almost a twelfth. 



In the clear still water of lakes and pools where, among the 

 roots of CyperacesB and other plants, there is little mud, and 

 where it also occurs among Sphagna, and in wet places in 

 moorish ground and on heaths it is equally beautiful, and nearly 

 of the same form, but less convex. There it does not attain a 

 greater length than a twelfth and a-half. 



With reference to specimens of this kind, and especially from 

 the Bishop's Loch, Mr. Jenyns says: "These appear identical 

 w r ith the Pisidium Jenynsii, and in every respect accord with 

 specimens from Battersea Fields and Cambridgeshire Fens." 



Such being the case, and the gradations being traced up to 

 the large individuals, there can be no doubt that all belong to 

 Pisidium Jenynsii ; which however presents greater differences 

 among individuals of its own species, than those usually seen 

 in what might be considered characteristic individuals of P. 

 Jenynsii and P. pulchellum, both which, and all their varieties, 

 I am almost persuaded, are of one single species. 



I have found the small and moderate, very clear, pinkish or 

 yellowish-white kind, very abundant, and most extensively 

 distributed in Aberdeenshire and Kincardineshire. It is in 

 fact one of the most common of all our fresh-water mollusca. 

 Miss Macgillivray has also brought it from Banffshire, and Mr. 

 Murray has found it in the district of Buchan. 



On the 22d July, the following observations were made on 

 numerous individuals found in the ditch mentioned above, 

 between Aberdeen and the Spital : 



When advancing in the water, the animal opens its valves 

 a little, places itself erect by means of the foot, which it gra- 

 dually protrudes, until it considerably exceeds the shell in 

 length. When thus extended it is of a linear-oblong form, 

 very little flattened, narrowed but rounded at the end. It 

 then contracts, and drags the shell quickly forward; after 

 which it is again extended, and again contracts. It is not 

 always stretched out in a direct line, but is moved in an undu- 

 lating manner, often from side to side, and appears to act as a 

 tentaculum, as well as an organ of motion. The siphonal tube, 

 which is at the same time extended, and kept so, is short, 

 cylindrical, truncate, and undergoes little alteration. A current 



