THE LAND AND FKESHWATEB MOLLUSCA OF DORSETSHIRE. 79 



AQUATIC. 



CLASS I. CONCHIFEBA, OB BIVALVES. 

 OEDEB LAMELUBKANCHIATA. 



FAMILY i. spn^Rnaas. 



, i 



Genus I.SPH^ERIUM, Scopoli. 



1. SPH.ERIUM CORNEUM, Linn., pl 5. 



CYCLAS OOENBA, F. $ ff., ii., p. 113, pi. xxxvii., 3, 4, 5, 6. 

 OAEDirni: COENEUM, Pult. eat., p, 31 ; Back, ed., p. 32, pi. 7, f. 2. 



Shell subglobular, nearly equilateral, thin, slightly transparent, 

 especially towards the umbones, yellowish horn-colour, surface 

 marked with indistinct longitudinal lines of growth, met 

 by very fine transverse striae, giving it a reticulated appearance ; the 

 inferior border has often a yellow band ; ligament scarcely visible on 

 the outside, as are the muscular and pallial scars inside. L.0.35. 

 B.0.45. 



Hab. Rivers, ponds, and ditches. Generally distributed. 

 2. s. RIVICOLA, Leach, pi. 5. 



CYCLAS EIVIOOLA, F. 8f IT., ii., p. 111., pi. xxxvii., f. 1, 2. 

 AJCNIOUM, Pult. cat., p. 31 ; Rack, ed., p. 32. 



Shell much inflated, suboval, nearly equilateral, solid, glossy, 

 yellowish horn-colour, with a few deeply -ridged dark zones, marking 

 the lines of growth ; anterior end rounded ; posterior end somewhat 

 produced and truncated; ligament short and conspicuous; muscular 

 and pallial scars fairly defined; nacrous white inside. L0.7. 

 B.0.9. 



Hab. Slow rivers and canals. Pulteney describes this shell 

 as not being uncommon in the Stour. Rackett considered it 

 "f a variety of Sphaerium corneum, and not to be confounded 

 with Tellina amnica." 



3. s. LACUSTRE, Muller, pi. 5. 



CYCLAS CALICDLATA, F. 8f H., vol. ii., p. 115, pi. xxxvii., f. 7 (as 



C. lacustris). 



Shell nearly globular, much inflated, compressed towards the mar- 

 gin, equilateral, thin, glossy and transparent ; greyish ash-colour^ 



