FAMILY LYMN.EACEA. 169 



Ancyhis fluviatilis. (Considerably Enlarged.) 



Genus IV. ANCYLTJS, Geoffroy. 



Animal ; ovoid and conical, carrying an overlapping light cap- 

 shaped shell, incurved at the posterior summit into a small 

 subspiral hook, which inclines either to the right or to the 

 left according to the position of the respiratory orifice. Mantle 

 very thin, not reflected over the edge of the shell. Head 

 broad, obtuse, with two rather distant slenderly triangular ten- 

 tacles, having the eyes at their inner base. 

 Shell ; orbicularly ovate and elevately convex, more or less radi- 

 ately ridged and striated, incurved at the posterior summit into 

 a hooked vertex, turning subspirally to the right ; or, oblong- 

 ovate, moderately convex and smooth, incurved a little pos- 

 terior to centre into a hooked vertex turning subspirally to 

 the left. 

 The existence in Europe of a small limpet adhering to stones and 

 to the stems and leaves of water plants in ponds, lakes, ditches, and 

 shallow streams, made it necessary to introduce the genus Ancylus 

 long before the animal had been observed. Lamarck placed the four 

 species known to him, two European, two "West Indian, at the end 

 of his marine water-breathing family of Calyptraciens, but " only 

 provisionally," said the acute conchologist. '•'According to the 

 observations of M. De Ferussac, the animal rises to the surface of 

 the water to respire air." The researches of subsequent naturalists 

 have shown that the animal has a similar organization of parts to 

 the animal of Physa, although taking the form of an ovoid conical 

 mass, secreting a simple Patella-like shell, with, however, a tendency 

 to coil in a very early stage of growth. 



There are two very distinct forms of Ancylus, one (Ancylastrum, 

 Moquin-Tandon), having the respiratory and other vital organs 

 situated on the left side of the animal, in which case the vertex of 

 the shell inclines to the right; another {Velletia, Gray), having the 



