46 ON THE STRUCTURE AND SYSTEMATIC POSITION OF CYSTOPELTA. 



produced into a slender flagellum ; and the vagina long and 

 slender and also invaginated upon itself. 



The features of this mollusk I am quite unable to reconcile with 

 the systematic position assigned to it by Try on in the " Manual 

 of Conchology, Series Pulmonata," Vol. I., p. 227. To me it is 

 clearly an aberrant form of Helicarion, and the following classifi- 

 cation would better describe its affinities : — Family, Zonitidce ; 

 Subfamily, Helicarionince ; Genus, Cystopelta. 



Col. Godwin- Austen describes how he traced from species to 

 species in the Indian Helicarionince a gradual diminution of the 

 shell from the helicoid test of Austenia to the rudimentary shell 

 of Girasia. The function of protecting the vital organs was 

 gradually usurped by the mantle which became thicker and 

 enlarged till the coalesced lobes exposed but a small portion of the 

 shell. The advantage to the animal in ability to squeeze itself 

 further and further into the crannies and crevices Avhere it loves 

 to hide is obvious, and the author speculates upon a further stage 

 when the shell shall have entirely disappeared and the united 

 mantle have entirely grown over it. This ideal form I believe we 

 actually possess in Cystopelta, and in our Australian fauna we 

 may say that as Helicarion is to Parmacochlea, so is the latter to 

 Cystopelta. 



No doubt Aneitea and Limax descended in a like manner from 

 shell-bearing ancestors, who have only transmitted to them the 

 little shapeless calcareous fragments concealed under the mantle 

 to prove their genealogy. Testacella too, by similar reasoning, 

 might claim some helicoid form like Rhytida for its origin in the 

 dim past. 



EXPLANATION OF PLATE. 



Fig. 1, — Spirit specimen of Cystopelta ■petterdi, Tate (?) ( x 4^). 



Fig. 2. — Rachidian, median and one lateral tooth of the odontophore of 



ditto (mag. 1000 diam.). 

 Fig. 3. — Jaw of ditto (much magnified). 

 Fig. 4. — Genital system of ditto (mag. 6 diam.). 



