146 1 ransacdons. — Zoology. 



Habitat: Throughout the whole of New Zealand; abundant in fresh 

 water, extending into brackish water. 



A very variable shell, but easily recognized by its smooth rounded 

 whorls. The aperture varies from nearly a half to a third of the length of 

 the shell. The narrow, turreted, form is P. zealandim, but I do not think 

 it is distinct, as there are many intermediate varieties. 



The animal is in all respects like P. corolla. The central lobe of the 

 median tooth is considerably produced. 



Our species appears to be quite distinct from the Tasmanian Paliidina 

 nigra (Q & Gr), the whorls in that shell, judging from the figure, being much 

 more oblique, and the aperture nearly parallel to the axis. Hydrobia 

 spelaa is, I think, only a small form of P. antipodum. It was found, along 

 with H. reevei, with moa bones in the Collingwood caves near Nelson. 



P. PUPOIDES. 



Plate I., figs. D and H. 



Shell minute, oval, olive-green, or brownish ; whorls 4, smooth, 

 rounded ; aperture ovate, two-fifths the length of the shell, peritreme 

 continuous. 



Formula of the denticles, -^ ; 9; 20; 30 to 35. 



Axis, -06 to -07 ; breadth, -035. 



Habitat: Estuary of the Avon and Heathcote rivers, in brackish water. 

 Not found in fresh water. 



This shell is very constant in its form, and very unlike the other species 

 of the genus, but the animal and dentition at once indicate its relationship. 

 No doubt it will be found in many other localities. The central lobe of 

 the median tooth is considerably produced ; the central denticle of the 

 first lateral is rather larger than the others. The inner surface of the 

 operculum has some small granules at its anterior end; these are not 

 calcareous. 



EXPLANATION OF PLATE I., FIGS. A to H. 

 A 1. Potamopijrgus cwningiana, x 5 times. 



2. ,, „ embryonic shells x 25 times. 



B 1. Potmnopyrgus corolla, x 5 times. 



2. „ ,, animal x 5 times. 



C 1. Potamopyrgus antipodum, three varieties X 5 times. 



2. ,, ,, operculum x 8 times. 



D Potamopyrgus pupoides, x 10 times. 

 E Potamopyrgus cumingiana, teeth x 470 times. 

 F Potamopyrgus corolla, median tooth x 470 times. 

 G Potamopyrgus antipodum, median and first lateral teeth x 470 times. 

 H Potamopyrgus pupoides, median, first and second lateral teeth x 470, and opercu- 

 lum from the inside x 20 times. 



