ANATOMY AND PHYSIOLOGY OF A SNAIL. 23 



radula with its numerous teeth rasps it off into the buccal cavity. 

 In some species, as Testacella, there is no beak at all, in others 

 it is either single, double, or treble. It also differs in contour, 

 and, according to these differences in shape or number, naturalists 



Fig. 6, Longitudinal section through the Bticcal Mass of Helix (after W. 

 Keferstein). O, mouth ; Mh, tuccal cavity ; M, muscles ; Rd, radula ; Kn, lingual 

 cartilage ; CE, oesophagus ; Kf, jaws ; Z, sheath of radula. 



have striven to form a system of classification, and none, perhaps, 

 more so than Moquin-Tandon and Dr. Morch. It has not, how- 

 ever, found as much favour with English workers as it deserves, 

 but on the continent it is almost universally adopted by concho- 

 logists, and is also, now and again, to be found referred to in 

 English works and papers on conchology. The student must, 

 therefore, make himself perfectly familiar with the terminology of 

 this portion of the subject, and the following synopsis is adopted 

 from an article communicated by Mr. Ralph Tate to the Intel- 

 lectual Observer about twenty years ago, but which contains, 

 nevertheless, a full explanation of the terms necessary for the 

 beginner to make himself conversant with. 



Synopsis of the genera of British Land and Fresh-water Mollusca, 

 grouped according to the nature of the mandibular organ. 



I. AGNATHA (without mandibles). Testacella. 

 II. MONOGNATHA (mandible single). 



1. Oxygnatha. Mandible lunate, smooth, rostrated. 

 Limax, Amalia, Vitrina, Hyalina. 



2. Elasmognatha. Horse-shoe-shaped, with a plate 

 behind. Succinea. 



3. Aulacognatha. Sides nearly parallel, striated. Butt- 

 mus, Cochlicopa, Clausilia, Balia, Pupa^ Vertigo, 



