[Illustration: Fig. 40. _Physa columnaris._ Paris Basin.] [Illustration: Fig. 41. _Melanopsis buccinoidea_; recent. Asia.] In regard to one of these, the _Ancylus_ (fig. 35.), Mr. Gray observes that it sometimes differs in no respect from the marine _Siphonaria_, except in the animal. The shell, however, of the _Ancylus_ is usually thinner.[29-A] [Illustration: Fig. 42. _Neritina globulus._ Paris basin.] [Illustration: Fig. 43. _Nerita granulosa._ Paris basin.] Some naturalists include _Neritina_ (fig. 42.) and the marine _Nerita_ (fig. 43.) in the same genus, it being scarcely possible to distinguish the two by good generic characters. But, as a general rule, the fluviatile species are smaller, smoother, and more globular than the marine; and they have never, like the _Neritae_, the inner margin of the outer lip toothed or crenulated. (See fig. 43.) [Illustration: Fig. 44. _Cerithium cinctum._ Paris basin.] A few genera, among which _Cerithium_ (fig. 44.) is the most abundant, are common both to rivers and the sea, having species peculiar to each. Other genera, like _Auricula_ (fig. 38.), are amphibious, frequenting marshes, especially near the sea. [Illustration: Fig. 45. _Helix Turonensis._ Faluns, Touraine.] [Illustration: Fig. 46. _Cyclostoma elegans._ Loess.] [Illustration: Fig. 47. _Pupa tridens._ Loess.] [Illustration: Fig. 48. _Clausilia bidens._ Loess.] [Illustration: Fig. 49. _Bulimus lubricus._ Loess, Rhine.] The terrestrial shells are all univalves. The most abundant genera among these, both in a recent and fossil state, are _Helix_ (fig. 45.), _Cyclostoma_ (fig. 46.), _Pupa_ (fig. 47.), _Clausilia_ (fig. 48.), _Bulimus_ (fig. 49.), and _Achatina_; which two last are nearly allied and pass into each other. [Illustration: Fig. 50. _Ampullaria glauca_, from the Jumna.] 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.). [Illustration: Fig. 51. _Pleurotoma rotata._ Subap. hills, Italy.] [Illustration: Fig. 52. _Ancillaria subulata._ London clay.]