HEY : FRESH WATER MUSSELS IN THE OUSE AND FOSS. 2)1 



is universal in the lower reaches of the river. The colour of the 

 shell is a clear rich olive or sepia, and the beak is much 

 protruded. It seems strange that the locks should separate forms 

 of shells as completely as they do, for two circumstances must be 

 borne in mind. The first is that a good deal of water passes 

 round by what is called the backwater, especially in flood time; 

 and the second, that the locks were once 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 case of many other 

 species of freshwater shells. Thus a particular form of SphcBrhivi 

 laacstre predominates only near Foss Islands, while another 

 species, Sphcermm 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 River 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 Yearsley Lock 

 occur some dwarfed and malformed specimens of Unio tumidus. 

 This malformation I venture to attribute to the effect of the water 

 rushing over the dam. Rapidly running water is always delete- 

 rious to the development of such shells as generally affect still 

 waters. Near Clifton Slope occur some very thin, but bright and 



