34 
often opened, and have not, of course, existed from a very remote 
time : so that we seem here to have a striking example of the 
readiness with which forms of life are restricted in distribution 
and affected in shape and colouring. The same restriction of 
distribution and modification of form is exemplified in the river 
Foss in the ease of many other species of freshwater shells. 
Thus a partiolar form of Bplmrium lacustre predominates only 
near Foss Islands, while another species, Hxjhc^Tmm ovale^ occurs 
nowhere within miles of York except in the tiny space between 
the Castle Mills Lock and Blue Bridge, and what is still more 
strange, had never been found either there or anywhere else in 
England till within late years. Mr. Jeffreys has a theory that 
it was imported from America, but no communication, so far as 
I am aware, has ever existed between our Biver Foss and any 
vessels which have crossed the Atlantic. The coal barges of 
the Foss would be a sorry sort of craft in which to stem the 
billows which roll between England and America, and I am 
not aware that anything larger has in modern times floated 
upon its waters, nor even that any American export is ever 
brought up the river. 
A few special forms of freshwater mussels deserve notice. Just 
above Tearsley Look occur some dwarfed and malformed speci¬ 
mens of Unio tumiclus. This malformation I venture to attribute 
to the effect of the water rushing over the dam. Bapidly running 
water is alv/ays deleterious to the development of such shells as 
generally affect still waters. Near Clifton Scope occur some very 
thin, but bright and clean, Anodontas, only small in size. They 
are clean, doubtless because of the absence of drainage, and they 
are small and thin because, not only is drainage absent, which 
often affords rich food, but the river is very clear of vegetable 
matter. Near the IJnion, there occur in the Foss, shells of Vnio 
tumidus much curved in form. They resemble Unio ynargaritifer 
in shape. It is curious that all our British Fnios and Anodontas 
have a tendency to assume this form under certain circumstances. 
These investigations, made over a space of a dozen years or 
more suggest a few general reflections. The first reflection bears 
on distribution; that subject which the genius of Wallace has 
rendered so deeply interesting. For the fact of so temporary 
