In nearly all land Pulmonata the jaw is single, and is placed behind the upper lip. If a common _Helix aspersa_ be observed crawling up the inside of a glass jar, or feeding on some succulent leaf, the position and action of the jaw can be readily discerned. It shows very black when the creature opens its mouth, and under its operation the edge of a lettuce leaf shows a regular series of little curved indentations, in shape not unlike the semicircular bites inflicted by a schoolboy upon his bread and butter. The jaw of _Helix_ (Fig. 107, B) is arched in shape, and is strengthened by a number of projecting vertical ribs. That of _Limax_ (A) is straighter, and is slightly striated, without vertical ribs. In _Bulimulus_ (C) the arch of the jaw is very conspicuous, and the upper edges are always denticulated; in _Orthalicus_ there is a central triangular plate with a number of overlapping plates on either side; in _Succinea_ (E) there is a large square accessory plate above the jaw proper. The form of the jaw is peculiar not only to the genus but to the species as well. Thus the jaw of _H. aspersa_ is specifically distinct from that of _H. pomatia_, and that of _H. nemoralis_ is distinct from both. Wiegmann has observed[321] that in young _Arion_, _Limax_, and _Helix_, the jaw consists of two pieces, which coalesce by fusion in the adult, thus indicating a stage of development in advance of the double jaw which is found in most of the non-pulmonate Mollusca. In all fresh-water Pulmonata there are two small accessory side plates besides the jaw proper (Fig. 107, F). [Illustration: FIG. 107.--Jaws of various Pulmonata: =A=, _Limax_ (_gagates_ Drap., Lancashire, × 15); =B=, _Helix_ (_acutissima_ Lam., Jamaica, × 15); =C=, _Bulimulus_ (_depictus_ Reeve, Venezuela, × 20); =D=, _Achatina_ (_fulica_ Fér., Mauritius, × 7); =E=, _Succinea_ (_elegans_ Riss., Aral District, × 30); =F=, _Limnaea_ (_stagnalis_ L., Cambridge, × 30).] Nearly all the non-carnivorous Prosobranchiata, land, fresh-water, and marine alike, are provided with two large lateral jaws. Many of these are sculptured with the most elaborate patterns, and appear to be furnished with raised teeth, like a file. In the Nudibranchiata the jaws are of great size and beauty of ornamentation (Fig. 109). [Illustration: FIG. 108.--Jaws of =A=, _Triton australis_ Lam., Sydney; =B=, _Ampullaria fasciata_ Reeve, Demerara; =C=, _Calliostoma punctulatum_ Mart., New Zealand; =D=, _Cyclophorus atramentarius_ Sowb., Sanghir; all × 15.] [Illustration: FIG. 109.--Jaws of =A=, _Chromodoris gracilis_ Iher., × 15; =B=, _Scyllaea pelagica_ L., × 7; =C=, _Pleurobranchus plumula_ Mont., × 10; =D=, _Pleurobranchaea Meckelii_ Lam., × 5/2.]