Part 9 (2/2)

[Ill.u.s.tration: Fig. 31. _Planorbis euomphalus_; fossil. Isle of Wight.]

[Ill.u.s.tration: Fig. 32. _Lymnea longiscata_; fossil. Hants.]

[Ill.u.s.tration: Fig. 33. _Paludina lenta_; fossil. Hants.]

The univalve sh.e.l.ls most characteristic of freshwater deposits are, _Planorbis_, _Lymnea_, and _Paludina_. (See figures.) But to these are occasionally added _Physa_, _Succinea_, _Ancylus_, _Valvata_, _Melanopsis_, _Melania_, and _Neritina_. (See figures.)

[Ill.u.s.tration: Fig. 34. _Succinea amphibia_; fossil. Loess, Rhine.]

[Ill.u.s.tration: Fig. 35. _Ancylus elegans_; fossil. Hants.]

[Ill.u.s.tration: Fig. 36. _Valvata_; fossil. Grays, Ess.e.x.]

[Ill.u.s.tration: Fig. 37. _Physa hypnorum_; recent.]

[Ill.u.s.tration: Fig. 38. _Auricula_; recent. Ava.]

[Ill.u.s.tration: Fig. 39. _Melania inquinata._ Paris Basin.]

[Ill.u.s.tration: Fig. 40. _Physa columnaris._ Paris Basin.]

[Ill.u.s.tration: 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 sh.e.l.l, however, of the _Ancylus_ is usually thinner.[29-A]

[Ill.u.s.tration: Fig. 42. _Neritina globulus._ Paris basin.]

[Ill.u.s.tration: 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.)

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig. 45. _Helix Turonensis._ Faluns, Touraine.]

[Ill.u.s.tration: Fig. 46. _Cyclostoma elegans._ Loess.]

[Ill.u.s.tration: Fig. 47. _Pupa tridens._ Loess.]

[Ill.u.s.tration: Fig. 48. _Clausilia bidens._ Loess.]

[Ill.u.s.tration: Fig. 49. _Bulimus lubricus._ Loess, Rhine.]

The terrestrial sh.e.l.ls 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 pa.s.s into each other.

[Ill.u.s.tration: Fig. 50. _Ampullaria glauca_, from the Jumna.]

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