144 Transactions.—Zoology. 
the inferior margin more or less trilobate. First lateral broad and ex- 
cavated in the middle, contracted into a long peduncle, the denticles nearly 
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, 
which is thickened on its outer margin. Number of transverse rows of 
teeth, 55 to 69. 
Formula of the denticles, Toner 9 or 11; 20 to 23; 380 to 40. 
The formula of the denticles differs widely from that of Bythinella, and 
approaches more nearly those of Stomatogyrus and Amnicola; but Potamo- 
pyrgus is readily distinguished from both these genera by the shape of the 
third lateral tooth. 
P, cumingiana and P. antipodum are both ovo-viviparous, and probably 
the other species are the same. The species inhabit both fresh and brackish 
water. They are very variable in form, and the only reliable character is 
the dentition. 
Key to THe Spectres. 
Shell with spines, whorls more or less carinated. 
Spines long ; first lateral tooth with 9 denticles .. .. P. eumingiana. 
Spines short ; first lateral tooth with 11 denticles .. ~. FP. corolla. 
Shell without spines, whorls rounded. 
Whorls 5 or 6; basal denticles 3-8 .. ee Si -. P. antipodum. 
Whorls 4; basal denticles 4-4 i Bs oe -. P. pupoides. 
P. cUMINGIANA. 
Plate I., figs. A and E. 
Paludestrina cumingiana, Fischer. Jour. de Conch. viii., 1860, p. 208. 
Paludestrina salleana, Fischer, 1.c. 1860, p. 209 (?). 
Melania corolla, Reeve. Conch. Icon. fig. 866; not of Gould. 
Shell ovate, thin, olive-brown; whorls 54 to 64, angulated ; a row of 
distant curved spines on the last two or three whorls, 10 to 17 spines on the 
body whorl. 
Formula of denticles, sai 9; 28 to 25; 26 to 80. 
Axis, *2 to -23 inch ; breadth, -12 to °1. 
Habitat: The northern part of the North Island and the valley of 
the Waikato. I have not seen it from Wellington, nor from any part 
of the South Island. Fresh-water only. 
The shell is very variable in shape, especially in the angle of the whorls, 
which is sometimes sharp, sometimes rounded, and sometimes absent 
altogether on the body whorl. The peritreme is usually continuous, but 
occasionally it is discontinuous in apparently adult shells. The lower side 
of the median tooth is trilobate, but the middle lobe is not conspicuously 
developed, and is sometimes sligit, 
