56 TRACHELIPODA. PALUDINA. 
Neritina concava. Plate 1V. fig. 28. Found in the 
London Clay at Muddiford, and Highgate Hill. 
This genus and Nerita are closely allied, and some attention is required to 
distinguish them. In the Nerita, the inner side of the outer lip is generally 
provided with numerous transverse teeth, or plice, and are, for the most part, 
stronger shells, being frequently striate, grooved, or tuberculate externally. 
The Neritine inhabit fresh waters; the species are met with in almost all 
countries, 
They occur fossil in the newer formations above the London Clay, and in 
the Upper Marine formations, and in the Woolwich beds; as also in the same 
strata of Germany and France: 
TRIBE IV.— PERISTOMIDA. 
Shell conoidal, or subdiscoidal, with the margins of the 
aperture united, provided with an operculum; animals 
JSluviatile, respiring in water, 
Genus LXX.— AMPULLARIA. — Lamarc&. 
Generic Character. — Shell globular, or globularly dis- 
coidal, or discoidal and umbilicate; spire short, the volu- 
tions ventricose ; aperture entire, oblong, oblique, and its 
length considerably exceeding its breadth ; operculum tes- 
taceous, annular, with its nucleus nearly central, but placed 
rather nearer the inner side, and covered by an olive green 
horny epidermis, and exactly fitting the aperture. 
Ampuliarta ambulacrum. Plate V. fig. 29. Found in 
the London Clay at Hordwell. 
The shells of many of this genus, and those of Planorbis, are so similar in 
external form, that considerable attention is necessary to distinguish them. 
Lamarck, the founder of this genus, has himself run into error in this respect. 
The chief difference in the discoidal species and the Planorbis consists in the 
aperture, and in being opercular. The aperture in Ampullaria is oblong, 
entire, and longer than it is wide, and is provided with a testaceous operculum ; 
in Planorbis the aperture is always transversely elongated, and destitute 
of an operculum ; besides, their spire is always reversed. The Ampullarie 
may also be confounded with the shells of Natica and Helex, especially when 
the shells have not the operculum to assist in determining them. They differ 
from Natica in being devoid of the testaceous callosity on the side of the 
umbilical region; and from Helix, in not having a thickened and revolute 
outer lip. 
Several of the species are reverse or heteroclitical shells. 
The Ampullariz inhabit rivers and lakes, in tropical climates, and are not 
numerous. 
Fossil species have been found in the Caleaire-grossier near Paris; at 
Hordwell in the London Clay ; and in the fresh water formation at Headon 
‘Hill, Isle of Wight. 
Genus LXXI. — PALUDINA. — Lamarck. 
Generic Character.— Shell ovate or oblong; spire 
