36 HALF-HOURS IN THE GKEEN TAKES. 



Equally well-fitted for the same place are the flat- 

 coiled shells, termed Planorbis, which abound in 

 various species, ranging in size from a pin's head to 

 the diameter of a shilling, on the under side of such 

 aquatic plants as the pond- weed (Potamoyetori). 

 The largest of these is Planorbta corneus, which has 

 about five whorls, with a broad, shallow hollow, or 

 umbilicus, on the under side. It is a capital 

 mollusc for clearing away the green scums that will 

 accumulate in the best kept aquaria, and is much 

 sought after for the purpose. In the summer time 

 you may find it in dried up ponds, with a thin film 

 spun across the mouth of its shell to prevent the 

 moisture of the animal being dried up, whilst the 

 creature itself is in a torpid condition. P. vortex is 

 a species very commonly distributed. It is a flatter 

 and thinner shell than the forementioned, and has 

 more whorls, the outer being, sharply edged or 

 keeled. This easily distinguishes it from P. spirorbis 

 which may be found in the same localities, for the 

 whorls of the latter are rounder, and the entire shell 

 is not so flat. P. albus is another very common 

 species, taking its name from the whitish colour of 

 the shell, by which, and by its five whorls, the last 

 one of which is disproportionately enlarged, it may 

 be readily identified. 



The Pcdudina, Bithinia, and Vcdvata have a dif- 

 ferent internal structure than the others mentioned. 

 This is especially noticeable in their breathing 

 jpgans, which are adapted for obtaining the air 



