270 HEY : FRESH WATER MUSSELS IN THE OUSE AND FOSS. 



tion that the Ouse specimens have a slightly curved'form, and 

 belong to the variety termed ciirvirostris. Between Castle Mills 

 Lock and Layerthorpe Bridge Anodontas are more or less 

 abundant, but they no longer present the beautiful radiated 

 colouring which distinguishes the Blue Bridge specimens. 

 Drainage affects the colouring unfavourably, but in certain parts 

 improves the size. There was, and perhaps is still, a spot where 

 warm water was discharged into the river. At this point the 

 shells were observed to be larger and more delicate than elsewhere, 

 exemplifying the general effect of heat upon forms of life. At 

 Layerthorpe Bridge shells cease to exist in the river. This is due 

 to the poisonous matter percolating through the banks from the 

 Gas Works. However, immediately above Monk Bridge, another 

 species of Uiiio appears, and is more or less abundant from that 

 spot up to Yearsley Lock. This species is Unio tiimidus; and what 

 is remarkable about it is that it appears in two forms — -one a 

 thick, dark brown wedge-shaped form ; the other a thinner, wider, 

 and greenish-tinted form. Why the same species should be 

 present in two forms under exactly the same circumstances is a 

 puzzle, for it is a generally received law that were two different 

 forms exist under the same conditions each has a right to be 

 elevated to the dignity of a species. And this is a law I should 

 like rigidly to adhere to, for I conceive it to be one of the few 

 thoroughly scientific criterions of a species. Near the bathing 

 place in the Foss, there exists, though it is very scarce, a curious 

 form of Unio, which in the "Journal of Conchology" I described 

 as Unio tumidus. I now believe it to be more correctly referred 

 to piciorum. The shell is large, very heavy, much truncated, and 

 in colour a dark olive brown. Above Yearsley Lock we find a 

 great change in the form of the toothless mussels. The form 

 cyg.nea, in which the sides of the shell are parallel, takes the 

 place of the crested form, which, according to my experience, 



J.C., iii., January, 1882 



