Page:Land Mollusca of North America (north of Mexico) Vol. II Part 2 (1st half).pdf/49

 ===Systematic Treatment (Continued)===

Family XIV. LIMACIDAE
Limacidae Gray, 1S24. Ann. Philos. (N.S.) 8:107, in part.--Hesse, 1926. Abh. Arch. Molluskenk., 2:7.

Aulacopod slugs with an oval mantle on the forward part of the back, and extending forward in a free lobe under which the head may be retracted. Shell reduced to a flat plate which usually is wholly covered (except in some genera of Milacinae and Parmacellinae). Foot with the band below pedal groove narrow; no caudal fossa; the sole tripartite, locomotion rhythmic, with direct waves over the middle field. Breathing pore in right margin of mantle, behind a short slit to the edge. Jaw smooth. Marginal teeth of radula simply thorn-shaped or bifid, with narrow, oblong basal-plates. Duct of the spermatheca short, usually opening directly into the atrium. Tentacular and buccal retractors converging and united posteriorly into a single band.

Distribution.--Western Palearctic region, one genus (Deroceras) extending also over northern Asia and North America.

Although Limacidae are superficially like the Arionidae, they are not directly related, having assumed the slug form independently, from different ancestors. The limaces differ in many details, such as the smooth jaw, the narrow basal plates of the marginal teeth, the union of the free retractor muscles into a single band posteriorly, the position of the pneumostome behind the mantle-cleft, the narrowness of the band below the pedal groove and the distinct waves of the sole in progression.

This family comprises about a dozen genera with a large number of species, nearly all native to Europe and adjacent parts of Asia and Africa. A certain number of adaptive species of the genera Limax, Deroceras (Agriolimax) and Milax have been spread over the world by commerce. Only Deroceras is known to have attained a wider range by natural means of dispersal, extending across northern Asia, over North and into South America.

The apparent absence of fossil limacid shells in loess1 of the Mississippi Valley or other interglacial deposits, together with the presence of strongly distinct species in the mountain and Pacific states, and in the Pliocene, lead to the following inferences: 1 Baker. F. C. 1920. Life of the Pleistocene, Univ. 111. Bull. 17. Limacid shells reported only from the Wabash, r. post-glacial formation.

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