292 CONCHIFERA. 



Mytilus avonensis. Linn. Trans, viii. t. 3. a. f. 4. 

 Anodonta crassa. Marks, MSS. 



Inhab. streams and ponds in calcareous countries. 

 Mr. Miller very justly remarks (Ann. Phil. iii. 

 377.), "I perfectly agree with Dr. Maton in consi- 

 dering M. avonensis only a variety of M. anatinus. 

 Miss Bennett of Norton House favoured me lately with 

 specimens from Tisbury, Wiltshire. They are old 

 shells, and the animal having lived in water highly 

 impregnated with chalk and calcareous matter, its 

 epidermis has been secreted so rapidly, and increased 

 the shell so much in thickness, that the Linnean cha- 

 racter, ' testa fragilissima,' is entirely lost." 



Var. 8. Shell small, elliptical-ovate, brittle, rather 



produced behind, short and rounder in front. 

 Anodonta anatina. Pfeiffer, i. 112. t. 6. f. 2.; 

 Schroet. Flussc. t. 1. f. 3. ; Rossm. Icon. t. 30. f. 

 417—420. 



Inhab. rivers, ponds, and ditches. 



A most variable species, which appears to assume 

 different appearances under every circumstance ; as, 

 for example, the depth, the stillness or motion, and 

 the purity or impurity, or peculiar impregnation of 

 the water in which it happens to be located. 



Mr, Alder considers A. cygnea, A. cellensis Pfeif- 

 fer, A. intermedia, and A. anatina Lam., and A. 

 ventricosa Pfeiffer, as distinct British species. (Mag. 

 Zoo!. §■ Bot. ii. 118.) 



Mr. Sheppard, after describing the four species, as 

 he considers the varieties of this species to be, sums up 

 as follows : — "To bring the specific differences above 

 enumerated into one point of view ; M. anatinus is 

 distinguished from M. cygneus by its anterior (pos- 



