ANADONTA. 15 



var. CELLENSIS. Gray. Shell larger, more ventri- 

 cose, thinner, and more brittle ; more produced behind, 

 and epidermis of a greener tint. 



Hal). Ponds in very still water. 



var. VENTRICOSA. Pfeif. Valves more solid and 

 swollen ; in shape, rhombic oval ; the epidermis being 

 often beautifully coloured. 



var. ANATINA. Pfeif. Smaller than the above ; 

 some of the lines of growth so indistinct as to show a 

 considerable breadth smooth and glossy ; hinder ex- 

 tremity broad and sub-rhomboidal ; dorsal line rising 

 behind so as to give it a winged appearance ; epidermis 

 a bright green with rayed lines. 



var. ROSTRATA. Pfeif. Rather rhombic ; much 

 produced behind, so as to form a sort of beak ; short 

 and rounded in front ; colour generally brownish. 



var. AVONENSIS. Gray. Very nearly connected 

 with the preceding, but more rounded; ventricose, 

 thick, solid, and slopes rather angularly behind. 



Hob. The localities of this species are so numerous 

 that it is less difficult to say where it is not, than where 

 it is found. Though lakes and ponds which have a 

 constant influx of fresh water, but possess, at the same 

 time, an unruffled surface, except when disturbed by 

 winds, seem most favourable to its perfect development, 

 yet running streams with sandy or gravelly bottoms 

 furnish a stunted race of less pleasing appearance. In 

 collecting this species (as in others which vary much), 

 the young conchologist should carefully label his speci- 

 mens with the locality, adding a few remarks on the 

 nature of the water, the character of the bottom, or any 

 other circumstance which could have any effect upon 

 the shell, or its inmate. 



MYTILID^E. 



DREISSENA. Van Beneden. 



Shell JEquivalve, very inequilateral, triangular ; character- 

 ized by a small transverse plate under the hinge within. 



