SHELL'COLORATION INT BRITISH EXTRA-MARINE MOLLUSCA. 65 



siderable and sudden variation of temperature, consequent on 

 shallow water, and to the action of torrents. This generally leads 

 to important modifications : free-swimming forms are lost, e.g. in 

 Anodonta and Astacus, where the Nauplius and Zooea stages are 

 entirely slurred over ; and in Hydra we find the medusiform 

 person reduced to a mere gonadial excrescence on the hydriform 

 person. Indeed, Prof. Sollas remarks,* " As Spongilla, however, 

 is a fresh-water form, anomalies in its development might almost 

 be expected." But the colouring of the shell cannot depend so 

 much on its physical, inorganic, as on its organic surroundings. 

 The only differences, which, I imagine, are thus produced, are in 

 the ground colour e.g. of " Planorbis " comeus, living in ferru- 

 ginous waters. Mr. E. W. Bowell assures me that here the 

 reddish colouring is found in the prismatic layer, not merely as 

 a superficial coating. 



It is very interesting to note, that of the five genera of more 

 brightly coloured British fresh-water mollusca, four are closely 

 connected with the sea. Neritina is found in fresh water, brackish 

 water, and salt water (and on land) ; Dreissena is closely allied to 

 Mytilus, and was undoubtedly recently a marine form, a fact 

 which is strongly attested by its still possessing the free-swimming 

 larva, which in Unio, &c, has been so largely modified ; t Unio 

 and Anodonta are nearly allied to sea forms. Paludina alone 

 offers considerable difficulties : its affinities with any recent 

 marine form are by no means obvious, and there seems good 

 evidence to show that the " elaborately coloured I forms arose 

 from simple and unornamented " ones.§ We have then here to 

 suppose that for some purpose not very clear (probably cryptic), || 

 this genus has been modified since its habitation in fresh water, 

 which has lasted for some time, Cretaceous onwards. The allied 

 British genera, Bythinia and Valvata, have the usual horn-coloured 

 shell, which also exists, somewhat strangely, in the brackish- water 

 genus Hydrobia. 



* Article " Sponges," ' Encyclopaedia Britannica.' Keprint, 1891, p. 52. 



f Korschelt, Sitzungsb. Naturf. Berlin. 1891, p. 131. 



| I do not think though that the colours in England ever reached 

 Da Costa's brown with red transverse stripes. See ' British Conchology,' 

 1779, t. vi. fig. 2. 



§ A. E. Wallace, "Darwinism," 1889, p. 381: ' Nature,' xiv. p. 275. 



Ii The green and brown stripes may resemble water-weeds on a reddish 

 or brown mud. 



