Almost all bivalve shells, or those of acephalous mollusca, are marine, about sixteen only out of 140 genera being fresh-water. Among these last, the four most common forms, both recent and fossil, are _Cyclas, Cyrena, Unio,_ and _Anodonta_ (see Figures); the two first and two last of which are so nearly allied as to pass into each other. Fig. 23: Gryphæa incurva. Lamarck divided the bivalve mollusca into the Dimyary, or those having two large muscular impressions in each valve, as _a b_ in the Cyclas, Fig. 18, and Unio, Fig. 22, and the _ Monomyary,_ such as the oyster and scallop, in which there is only one of these impressions, as is seen in Fig. 23. Now, as none of these last, or the unimuscular bivalves, are fresh-water,[2] we may at once presume a deposit containing any of them to be marine. Fig. 24: Planorbis enomphalus. Fig. 25: Limnæa longiscala. Fig. 26: Pauldina lenta. Fig. 27: Succinea amphibia. Fig. 28: Ancylus velletia. Fig. 29: Valvata piscinalis. Fig. 30: Physa hypnorum. Fig. 31: Auricula. Fig. 32: Melania inquinata. Fig. 33: Physa columnaris. Fig. 34: Melanopsis buccinoidea. Fig. 35: Neritina globulud. Fig. 36: Nerita granulosa. The univalve shells most characteristic of fresh-water deposits are, _Planorbis, Limnæa,_ and _Paludina._ (See Figures.) But to these are occasionally added _Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, Potamides,_ and _ Neritina_ (see Figures), the four last being usually found in estuaries. Fig. 37: Potamides cinctus. Some naturalists include _Neritina_ (Fig. 35) and the marine _Nerita_ (Fig. 36) 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 _Neritæ,_ the inner margin of the outer lip toothed or crenulated. (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 amphibious, 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 recent and fossil state, are _ Helix_ (Fig. 38), _Cyclostoma_ (Fig. 39), _Pupa_ (Fig. 40), _Clausilia_ (Fig. 41), _Bulimus_ (Fig. 42), _ Glandina_ and _Achatina._ Fig. 38: Helix Turomensis. Fig. 39: Cyclostoma elegans. Fig. 40: Pupa tridens. Fig. 41: Clausilia bidens. Fig. 42: Bulimus lubricus. _Ampullaria_ (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 considered by conchologists to belong to _Natica_ and other marine genera.