160 MYTILIDA. 
Var. I. spléndens (Mog.). L.emon-coloured, larger. 
Var. II. globosa (L. E. Ad.). Spheeroidal. 
sm@ P. MiLIUM (razz of millet) ffeld. P\. IX., f. 9, & Pl. X., f. 3:5 
[=P. rosEuM /Jef7.] 
Somewhat oblong, tumid, glossy, regularly striated concentrically ; 
ligament almost invisible. L. 2} mm. B. 3} mm. 
Its habitats are similar to those of the last species. Its 
distribution is more restricted than any of the others, it 
-having been found in Sligo only, in Ireland; in Wales 
only in Anglesea. I have taken it in the northern, south- 
ern, and western and midland counties of England, but 
it is more common in Scotland than south of the Tweed. 
Its oblong shape and straight lower margin serve to 
distinguish it from its brethren. 
Var. alpéstris (Clessin). Very strongly striated, very 
globose, somewhat produced ; lower margin towards pos- 
terior extremity more bent up and more arched. 
S2b-order.—HETEROMYA. 
Famtily.—MYTILIDe. 
Genus.—DREISSENSIA Van Beneden. 
Boat-shaped, furnished with a byssus ; deaks placed at the extreme 
end ; /igament internal. 
D. POLYMORPHA (many shaped) Pallas, P\. VIIL., f. 3. 
Shell triangular, sharply keeled, lower margin incurved ; solid ; 
ligament long and narrow; azzmal furnished with a dyssus. L. 
35mm. 8B. 15 mm. 
