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variety termed ciirvirostris. Between Castle Mills Lock and 
Layerthorpe Bridge Anodont&s are more or less abundant, but 
they no longer present the beantifnl 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 Gfas Works. 
However, immediately above Monk Bridge, another species of 
Unio appears, and is more or less abundant from that spot up to 
Yearsley Lock. This species is Tlnio tumidus ; 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 where 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 pictorum. 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 cygnea, in which the sides of 
the shell are parallel, takes the place of the crested form, which, 
according to my experience, 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, es¬ 
pecially in flood time; and the second, that the locks were once 
