ANCYLUS. 257 



The upper disc in the many-whorled shell of 

 P. vortex is deeply concave; while in P. spirorUs, a very 

 similar species, both discs are concave. The whorls of 

 P. contoHus are very closely coiled^ and the aperture is 

 semilunar. 

 P. nitichs and P. lacustris have small, highly-polished shells, 

 with angular whorls nearly overwrapping each other. 

 The latter species has internal laminae, represented ex- 

 ternally by dark lines. 

 Ancylus, or Fresh-ivater Limpet. — This little animal, like 

 the Limnmis, has triangular tentacles. The shell is Kke a 

 limpet, having no spire, but the apex a little turned on one 

 side : in A. fluviatilis it is turned to the right, and in A. oh- 

 longus, which is long and narrow, it turns in the opposite 

 direction. 



It is truly wonderful to observe the opulence of nature, 

 and to notice in the different shells of these very similar 

 animals, Planorhis, Limnoius, and Ancylus, in what different 

 ways the same end may be answered. 



