THE LAND AND FRESHWATER MOLLUSCA OF DORSETSHIRE. 91 



Hab. On aquatic plants in running brooks, as well as in 

 slow rivers and ditches. River, Chamberlayne's, Bere Regis ; 

 Fordington meadows ; Stour, Sturminster Marshall ( J.C.M.P.). 



Physa f ontinalis, in common with some other water molluscs, 

 such as Limnaa glutinosa, L. stag nulls, Bythinia tentaculata, have 

 the power of thread-spinning for suspension. Montagu, at the 

 beginning of this century noticed this habit in Phyta fontinalis. 

 He says : " B alia f ontinalis has a very considerable locomotive 

 power, and transports itself, by adhering to the surface of the 

 water with the shell downwards, against which it crawls with as 

 much apparent ease as on a solid body ; and will sometimes let 

 itself down gradually by a thread affixed to the surface of the 

 water." 



In the Quarterly Journal of Concholoyy for November, 1878, on 



"Molluscan Threads," by Mr. Sheriff Tye, speaking of Physa 

 hypnorum, he says: "In one case I saw three Physce and a 



Limncea glabra upon the thread of a former at one time. Often 

 when two Phi/see meet on the same thread they fight as only 

 molluscs of this genus can, and the manoeuvres they go through 

 upon their fairy ladders outdo the cleverest human gymnast that 

 ever performed. I once saw one ascending, and when it was 

 overtaken by another, then came the tug of war. Each tried to 

 shake the other off by repeated blows and jerks of its shell, at 

 1 the same time creeping over each other's shell and body in a 

 most excited manner. Neither being able to gain the mastery, 

 one began to descend, followed by the other, which overtook it, 

 reaching the bottom first." 



Genus III.LINNMA (LYMNEAJ, Bruguiere. 

 1. LIMN.EA GLUTINOSA, Midler, pi. 7. 



Body black OP greenish grey with a yellow tinge. 



Shell oval-globose, inflated, extremely thin and fragile, almost 

 membraneous, very glo.-sy and transparent, pale horn-colour, with 

 rather distinct, finely-marked striae ; -whorls three or four, the last 



