HEY : FRESH WATER MUSSELS IN THE OUSE AND FOSS. 269 



toothless. There are three kinds of Unio found in England^ 



firstly the famous pearl mussel, which is confined to mountain 



streams, and therefore does not occur at York; secondly, Unio 



plctoritm, so-called because painters used the shell for palettes; 



and thirdly, Unio tutnidus. The last two occur in both our rivers. 



In the genus Anodojita, Jeffreys makes two species, Anatina, in 



which tho hinge line is carried into a sort of crest, and Cygncea. 



which has the hinge line almost parallel with the other edge of 



the shell. This distinction, however, is difficult to preserve: one 



form passes by imperceptible grades into another. Both forms 



occur abundantly at York. The mussels can be collected in 



plenty only when the locks are open. The water then sinks 



sufficiently low to disclose large numbers of them sticking in the 



soft banks. When Naburn Lock is opened that small portion of 



the River Foss (only some 200 yards in length) which lies between 



Castle Mills Lock and the Blue Bridge is almost drained. Very 



near the mouth of the Foss occurs a number of mussels belonging 



to the species Anodonta anati?ta. They are remarkable for 



possessing a beautiful ornamentation of rich green rays. The 



epidermis is lustrous, the interior highly nacreous. Now, by 



walking a few yards we find ourselves following the bank of the 



Ouse. Here occurs the very same form of Anodonta, but how 



changed in appearance! Instead of a lustrous green epidermis, 



the shell is of a dark dead brown colour, the pearliness of the 



interior is quite dull, and the phenomenon of erosion, or the 



eating away of the epidermis and upper layers of the shell, is 



extensively developed. Erosion is caused by the presence of 



carbonic acid or by the rapidity of the current of water. Probably 



the former has caused the disfigurement of the Ouse Anodontas, 



owing to the drainage matter present in the river just below York. 



Exactly the same contrast is shown in the specimens of Unio 



pictorum, from the same two localities, with the additional distinc- 



