THE LAND AND FRESHWATER MOLLUSCA OF DORSETSHIRE. 81 



Hab. Slow streams and standing water ; it is a British Ter- 

 tiary fossil. Var. I. Hoi well (Rev. H. H. Wood). Var 2. 

 Weymouth (Damon). Var. 3. River Stour, Spettisbury 

 (J.C.M.P.). 



3. P. PUSILLUM, Gmelin, pi. 5. 



Shell oval, rather ventricose, nearly equilateral, thin, with finely 

 marked, unequal concentric striae, yellowish white or ash-colour; 

 anterior and posterior sides rounded, the latter being slightly more 

 produced than the other ; inferior border arched ; umbones inflated ; 

 beaks blunt ; ligament not visible from the outside ; muscular and 

 pallial scars well marked. L.0.175. B.0.2. 



Hab. Ponds and ditches. Ditches, Stoborough meadows, 

 near Wareham ; ditches adjoining the beach, between Abbots, 

 bury and East Bexington ; watercourses, Sturminster Marshall 

 (J.C.M.P.). 



4. P. NITIDUM, Jenyns, pi. 5. 



Eobinson's Me of Pur beck, p. 178. 



Shell suborbicular, ventricose, nearly equilateral, with fairly 

 denned transverse, concentric striae, becoming deeply grooved 

 towards the umbonal region ; thin, very glossy and iridescent, horn- 

 colour varying in shades ; anterior side rounded, posterior side slightly 

 produced sloping downwards and rounded. L.U.15. B.0.15. 



Hab. Lakes, ponds, and standing water. Wilkeswood 

 quarry, Langton Matravers, and in several small ponds in the 

 neighbourhood of Swanage (R. H. Soden Smith) ; Chapman's 

 Pool, near Encombe ( J.C.M.P.). 



5. ROSEUM, Scholtz, pi. 5. 



Damon's Geology of Weymouth, p. 233. 



Shell oblong ventricose, thin, very glossy, with deep, regular, 

 concentric striae, yellowish horn-colour ; anterior end truncate, slop- 

 ing abruptly below ; posterior end elongated and rounded, lower 

 margin nearly straight ; beaks prominent, obtuse and placed con- 

 siderably on one side ; ligament inconspicuous. L.O.I. B.0.15. 



Hab. Marshes, ponds, ditches, and stagnant waters. 

 FAMILY II. UNIONID^. 



Genus I. UNIO, Philippswn. 



Closely allied to the marine Mussels, but differing in the struc- 

 ture of the foot, which is greatly developed in dimensions. All 

 comprising this genus inhabit freshwater. 



