312 Introduction to Aninial Morphology. 



debarda. 3. Helicidse — herbivorous; shell external, spiral; 

 body separated from the foot ; jaws strong, crescentic. Helix, 

 the snail, has a complete peritreme with separate borders, 

 modified by the projection of the whorl before the last ; its 

 1800 species form about fourteen sub-genera. Bulimus has 

 oval shells, with unequal margins to the peritreme, and in- 

 cludes 1200 species. Achatina is Bulimoid, non-umbilicated, 

 with an open posterior end and a sharp edge. Vitrina has a 

 glass-like shell with a simple mouth. Nanina is snail-like, 

 polished, umbilicated, depressed, with a bilobed mantle. 

 Zonites is umbilicated, multispiral, with a tail gland. Acha- 

 tinella is often dexiotrope, like the slender cylindrical 

 Clausilia. Pupa is ovate or cylindrical, with a toothed or 

 plane peritreme, and with tentacles, small, anterior, or none 

 (Vertigo). 



Sub-order 3. Basommatophora — eyes posterior, or internal 

 to the two tentacles, sessile, including: — i. Limnaeidae — 

 shell horny, translucent, with a large body whorl, a sharp lip, 

 often retroverted (Anphipeplea), rarely dexiotrope (Physa), 

 discoidal (Planorbis), or patelliform (Ancylus), or neritiniform, 

 with a wide mouth (Latia). 2. Auriculidae — shell thick, with 

 a thick periostracum ; inner lip folded ; jaws strong ; the 

 spine may be much elevated (Carychium), the outer lip sharp 

 or thick. Allied to this is the patelliform genus Siphonaria, 

 which has a gill in the pulmonary cavity, and is thus a passage 

 form. Pulmonates have been arranged according to their 

 jaws, by Morch, into six groups: — i. Oxygnatha, or those 

 with a sharp tooth on the jaw, as Limacidae and Vitrinidae. 

 2. Agnatha — ^jawless, asTestacellidaeandOnchidiidae. 3. Aula- 

 cognatha — with striped jaws, crenated at the edge, as Pupa, 

 Clausilia. 4. Odontognatha — ^jaws with marginal rows of 

 teeth, as Arionidae, Helicidae (except Pupa, &c.) 5. Gonio- 

 gnatha — with superficially striated jaws, as Orthalicidae, 

 Veronicellidae, Janellidae, Succineidae. 6. Polygnatha — with 

 more than two jaws, as Limnaeidae. 



