208 Transactions. — Zoology. 



species, of which six are univalves and two are bivalves. We 

 have thus, roughly speaking, a third of the number of fresh- 

 water shells ranging through our Islands. Dividing the 

 univalves again, we have two of the river-snails, or Hydro- 

 biidce, and four of the pond-snails, or LimnceidcB. 



To understand the distinction between these river-snails- 

 and pond-snails, let me take you down to the mouth of 

 Sturm's Gully for a few minutes. You will find the rocks 

 of the beach there studded with the shells of two relatives of 

 the English periwinkle — one, Littorina cincta, a small edition 

 in shape and colouring of the English shell ; the other, 

 Littorina cmrulescens , a dirty- white shell with a broad band 

 of slate-blue running round its turns or whorls. Pick one off 

 the rock, and you will find it quickly shuts up the entrance of 

 its shell with a neatly fitting horny cap or lid, the operculum. 

 If you look about in the fennel-jungles of the cliff-bottom you 

 will find plenty of that handsome familiar garden-pest, the 

 common snail. Notice that as he drags himself, bubbling, 

 into the recesses of his shell he has no operculum. The 

 periwinkles have gills, breathing water ; the snails have 

 lungs, breathing air. Now, the river-snail is, broadly speak- 

 ing, a periwinkle that in the course of ages has taken to the 

 fresh water instead of the salt — in fact, "river-periwinkle" 

 would be a much more appropriate name for it ; the pond- 

 snail is a relative of the snail that has taken to fresh water 

 in preference to a land-existence, still retaining, however, its 

 lungs. 



'o 1 - 



Kivee-snails (Hydkobiid^;). 



Our two species of river - snails are Potamopyrgus anti- 

 jjodarum and Potamoi)yrgus corolla. They are both very com- 

 mon in all our creeks, lakes, and pools — in fact, in any of our 

 permanent water. Drag out a bundle of duckweed, water- 

 cress, or any floating herbage or timber, and you will be sure 

 to find it studded with tiny black-brown spirals, varying in 

 size from young shells the size of a pin's head to adults up to 

 Jin. in length. As a rule, in the more rapid creeks they are 

 under this size, but in the still mud-bottomed pools of our 

 narrow flats they come well up to this measurement. 



Potamopyrgus antipodarum is by far the commonest. You 

 will see it is not unlike its sea-going relative, the brown peri- 

 winkle — the same rounded whorls, with the mouth of the shell 

 closed by an operculum. 



Potamopyrgus corolla is a more striking shell than the pre- 

 ceding species, the whorls being angled or turreted, and along 

 the angle of the last three whorls is a row of spines. These 

 spines are well developed in the shells from still water, but in 

 our rapid creeks they are worn down to short stumps or absent 



