The Ampullaria ([fig. 50.]) is another genus of shells, inhabiting rivers and ponds in hot countries. Many fossil species have been referred to this genus, but they have been found chiefly in marine formations, and are suspected by some conchologists to belong to Natica and other marine genera.
All univalve shells of land and freshwater species, with the exception of Melanopsis ([fig. 41.]), and Achatina, which has a slight indentation, have entire mouths; and this circumstance may often serve as a convenient rule for distinguishing freshwater from marine 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 Ampullaria and the land shells ([figs. 45]-[49.]), when its outline is not interrupted by an indentation or notch, such as that seen at b in Ancillaria ([fig. 52.]); or is not prolonged into a canal, as that seen at a in Pleurotoma ([fig. 51.]).
Fig. 51.
Pleurotoma rotata. Subap. hills, Italy.
Fig. 52.
Ancillaria subulata. London clay.