FAMILY 5. LYMNAANA. 117 
Planorbis turritus, Miller. 
Bulimus hypnorum, Bruguiére. 
Bulla turrita, Gmelin. 
Physa hypnorum, Draparnaud. 
Physa turrita, De Férussac. 
ANCYLUS, Geoffroy. 
esta tenuis, p eformis, obliqué conica, vertice subacuto, posticé in- 
Testa te atelleetor blique rtice subacuto, post 
curvo ; apertura ovali, amplissima, margine undique simplici. 
‘It is more difficult,” says Deshayes, ‘‘ to determine the proper situa- 
tion of the Ancyli than that of any other genus in the system.” They 
were associated by Linnzeus with the Patelle; and although Geoffroy, 
one of the early French naturalists, is entitled to the credit of distinguish- 
ing them as a particular genus, they were still arranged in the same situ- 
ation as by his contemporary. The nature and true organization of these 
mollusks were indeed long unknown to naturalists ; they were guided, 
therefore, in their distribution of them by the patellaform structure of the 
shell. Miller, for example, adopted the genus Ancylus of Geoffroy, but 
still placed it in the same natural division with Patella ; De Montford, 
again, referred the Ancyli to his genus Helcion ; and even Lamarck, con- 
sidering the shell of Ancylus as a modification of that of Pileopsis, in- 
cluded the genus in his family of ‘ Les Calyptraciens,’ though not without 
considerable hesitation. De Férussac appears to have been the first to 
notice that the Ancyli, though aquatic, are true air-breathing mollusks ; 
he observed, that though living in water, they are compelled to rise to the 
surface in order to respire, like the rest of the Lymnzana, and we are in- 
debted to our countryman Guilding for the complete history of their ana- 
tomy. 
The shell of Ancylus may be described as being thin, patelleform, 
and obliquely conical; the vertex is rather sharp, and incurved poste- 
riorly ; the aperture is oval and very large, with the margin everywhere 
simple. 
