SPHARIUM. 153 
The latter theory may be put aside when it is remembered 
_ that the two species are often found separately, though the 
young of cygzea do resemble adult avatine and the animals 
of the two species cannot be distinguished by dissection. 
Var. I. ventricdsa (C. Pfr.). Larger, more solid. 
Var. II. complandta (Rossm.). Oval, compressed, beaks 
close to anterior margin. 
Var. III. vadidia (Jef.). With green and yellow 
radiations. 
Family.—SPHERUDE. 
Genus.—SPHERIUM Scop. 
Shell nearly equilateral; azzzal with two siphons. 
I. S. CORNEUM (horn coloured) Linné. P\. IX., f. 1. 
Somewhat globular; thin, glossy, opaque, yellowish horn-colour 
with paler bands indicating the lines of growth, finely and evenly 
striated in the same direction and also faintly striated from beak to 
margin; /igament short; Azmge furnished with a double cardinal tooth 
in each valve, 2 lateral teeth in the right and 4 in the left valve; 
muscular scars faint. L. 84mm. B. 114 mm. 
This common shell is found at the roots of flags and 
weeds and in the mud in ponds, ditches, canals and rivers 
in every part of the British Isles. It has the power of 
suspending itself in the water by threads of mucus. 
Var. I. flavéscens (McGill). Pale yellow. 
Var. II. nicleus (.Stud.). Smaller, nearly spherical. 
Var. III. scaldidéna (Norm.). Ovate, paler than the type. 
