BOYCOTT : THE HAUITATS OF FRESHWATER MOLLUSCA. 245 



ring in no less than 40 of the 86 ; and albus lives in 20 ponds as 

 well as in the lakes, streams and the river. As a parallel among 

 plants we may note that Potainogelon lucens has the same distribution 

 as auricuiaria, while /\ natdtis is like peregra and has been seen in 

 42 ponds. Maps of the distribution of each species indicate, with 

 the exception of P. coinplanaius^ a random topographical occurrence 

 which further diminishes the apparent importance of dispersion as 

 contrasted with habitat. 



As is evident from the table the different species ^ fall into more 

 or less definite habitat groups. 



{a). We have in the first place a series of nine species which are 

 common to all groups of habitats : — 



LimncBa peregra, Planorbis alb us ^ 



L. paiustris, P. complanatus, 



L. stagnalis, P. fontanus, 



Ancylus lacustris, P. vortex. 



Sphcerium corneuvi, 

 Of these, peregra, paiustris, stagnalis, complanatus and vortex appear 

 to prefer habitats other than closed ponds. Paiustris occurs in very 

 good habitats (river) or in very bad ones (drying roadside puddles) 

 and considering this flexibility it is wonderfully uncommon. 



(b). The second obvious series is the list of twelve species which 

 are absent from closed ponds and, with the exception of P. contortus, 

 from running ponds also : — 



Limncea auricuiaria, Bithinia tentaculata, 



Planorbis corneus, B. leachii, * 



P. contortus, Valvata piscinalis, 



Ancylus fluviatilis, V. crista ta, 



Physa fontinalis, Pisidium amnicum, 



Anodonia cygnea. Unto pictorum. 



Among these we may distmguish several subsidiary groups. 



(a). Anodonta, Unio and auricuiaria are confined to the river and 

 lakes. The occurrence of the larger bivalves is a special case, since it 

 is conditioned by the occurrence of fish on which they may achieve 

 the parasitic phase of their development. Whether any particular 

 kinds of fish are necessary for the naiads with which we are concerned 

 I do not know. But what is immediately germane, Mr. Latter, as he 

 has been good enough to tell me, has succeeded in the difficult 

 experiment of breeding A. cygnea through its metamorphosis on the 

 stickleback. We have, therefore, definite evidence that that fish will 



1 1 here omit the smaller Fisidia fur rtiisons I have already indicated ; it may be signifi- 

 cant iii3kl puicheilum and hcnslowanum have been found only in the river area, and that this has 

 not produced /<r^<>«a^«;« and obtusaU. There is evidently a fine field for enquiry here. 



