BY W. F. PETTERD. 69 



Of this shell I have examined a great number of specimens, 

 and I have invariably found it constant in its specific 

 characters. Its acutely keeled periphery, and remarkably 

 swollen and angled aperture, at once separates it not only 

 from our other forms, but also from all the known Australian 

 representatives of the genus. In colour even it differs from 

 the other Tasmanian kinds ; for it is always of an extremely 

 pale greenish horn, almost white. It is commonly found 

 attached to the leaves of aquatic plants, sometimes in 

 swiftly running water ; at Clynevale it is very plentiful. 



POTAMOPYRGUS, StIMPSON. 



Shell, ovate-conic or oval, imperforate ; body whorl more 



than half the length of the shell ; aperture ovate, the outer 



lip acute ; peritreme continuous or discontinuous. Opercidum 



horny, subspiral, without any internal px-ocess. Animal with 



the foot rather short, slender, taj^ering and pointed. Eyes 



on very prominent tubercles. Dentition. Median tooth 



trapezoidal, the inferior margin more or less trilobate. First 



lateral broad and excavated in the middle, contracted into a 



long peduncle, the denticles neai-ly equal. Second lateral 



pointed at the inner extremity ; the shank broad, and 



thickened on its outer margin. Third, lateral with the inner 



extremity broad and rounded, constricted at its junction with 



the very broad shank, whicli is thickened on its outer margin. 



Number of transverse rows of teeth, 55 to 69. 



Formula of the 7 or 9 ^ -n r,rv oo o^^ ^/^ 



, .. , -o 1 — o T -9 or 11; 20 or 23; 30^0 40. 



denticles, S or 4 — 3 or 4 ' ' ' 



The formula of the denticles differs widely from that 

 of Bythnella, and approaches more nearly those of Stomafo- 

 fjTjriis Sind Amnicola ; but PofainopTjrr/ us is readily distinguished 

 from both these genera, by the shape of the third lateral 

 tooth. 



The above is the diagnosis of this genus, as given by 

 Professor F. W. Hutton, in his paper on the New Zealand 

 Hydrohiint' ; it is a slight modification of Dr. Stimj^son's 

 original description that was found necessai*y on more 

 extended investigation. 



The distribution is given in Try on' s " Structural and 

 Systematic Conchology," as New Zealand and Cuba. 



PoTAMOPYEous NIGRA Quoy and Gaimard. 



Plate III. Figs. 2 to 8. 



Pahulina nigra, Qxioy and Gaimard. Voy. Astrolabe, iii., 



p. 174. 



Bishynia Legrandi, Tasmanica andunirari)iafa. Tenison-Woods. 



Pro. Royal Soc. Tas., 1867. 



Trans. New Zealand Institute, 1882. 



