56 
ELEMENTS OF GEOLOGY. 
Fig. 3T. cies are smaller, smoother, and more globular than 
the marine; and they have never, like the Neritm^ 
the inner margin of the outer lip toothed or crenu- 
lated. (See Fig. 36.) 
The Potamides inhabit the mouths of rivers in 
warm latitudes, and are distinguishable from the 
marine Cerithia by their orbicular and multispiral 
opercula. The genus Auricula (Fig. 31) is amphib¬ 
ious, frequenting swamps and marshes within the 
influence of the tide. ♦ 
The terrestrial shells are all univalves. The 
most important genera among these, both in a re¬ 
cent and fossil state, are Helix (Fig. 38), Cyelosto- 
ma (Fig. 39), Pupa (Fig. 40), Clausilia (Fig. 41), 
Balimus (Fig. 42), Olandina and Achatina, 
Ampidlaria (Fig. 43) is another genus of shells 
inhabiting rivers and ponds in hot countries; Many 
fossil species formerly referred to this genus, and which have 
been met with chiefly in marine formations, are now consid- 
Potamides 
cinctus, Sow. 
Paris basin. 
Fig. 38. 
Fig. 39. 
Fig. 40. 
Fis:. 41. 
Fig. 42. 
Helix Turonensis, Desh.; 
Faluns, Toiiraine. 
Cyclostoma 
clegans, 
Miill. ; 
Loess. 
Pupa 
tridenSf 
Drap.; 
Loess. 
Clausilia Bulimus luhricus^ 
hidenSf Miill.; Loess, 
Drap.; Ehine. 
Loess. 
ered by conchologists to belong to Natica and other marine 
genera. 
Fig. 43. 
All univalve shells of land and fresh-water species, with 
the exception of Melanopsis (Fig. 34), and Achatina^ which 
has a slight indentation, have entire mouths; and this cir¬ 
cumstance may often serve as a convenient 
rule for distinguishing fresh-water from ma¬ 
rine strata; since, if any univalves occur of 
which the mouths are not entire, we may 
presume that the formation is marine. The 
aperture is said to be entire in such shells 
as the fresh-water Ampullaria and the land- 
shells (Figs. 38-42), when its outline is not 
interrupted by an indentation or notch, such 
as that seen at h in Ancillaria (Fig. 45); oi¬ 
ls not prolonged into a canal, as that seen at a in Pleurotoma 
Ampullaria glauca, 
from the Jiimna. 
