VI MANTLE REFLECTED OVER THE SHELL 1/5 



appearance altogether. In Vitrina and some allied genera we 

 have a type in which the mantle lobes are partly reflected over 

 the shell, which at the same time exhibits rather less of ^a spiral 

 form than in Helix. In the stage represented by Parmacella^ 

 the mantle edges have coalesced over the whole of the shell, 

 except for a small aperture immediately over the spire ; the 

 nucleus alone of the shell is spiral, the rest considerably flat- 

 tened. In Limax the shell has become completely internal, and 

 is simply a flat and very thin plate, the spiral form being 

 entirely lost, and the nucleus represented by a simple thicken- 

 ing at one end of the plate. In Avion, the final stage, we find 

 that the shell, being no longer needed as a protection to the 

 vital organs, has either become resolved into a number of inde- 

 pendent granules, or else has entirely disappeared. 



Some indications of a similar series of changes occur in the 

 Pelecypoda. The mantle edge of Lepton is prolonged beyond 

 the area of the valves, terminating in some cases in a number 

 of filaments. In G-aleomma and Scintilla the valves are par- 

 tially concealed by the reflected mantle lobes, and in a remark- 

 able form recently discovered by DalP (^Chlmnydoconeha) the 

 shell is completely imbedded in the mantle, which is perforated 

 at the anterior end by an orifice for the mouth, and at the pos- 

 terior end by a similar orifice for the anus. In all these cases, 

 except Lepton, it is interesting to notice that the hinge teeth 

 have completely disappeared, the additional closing power 

 gained by the external mantle rendering the work done by a 

 hinge unnecessary. It is quite possible, on the analogy of the 

 Gasteropoda mentioned above, and also, it may be added, of 

 the Cephalopoda and other groups, that we have here indicated 

 the eventual occurrence of a type of Pelecypoda altogether de- 

 prived of valves, a greatly thickened mantle performing the part 

 of a shell.2 



The following works will be found useful for further study 

 of this portion of the subject : — 



F. Bernard, Recherches sur les organes palleaux des Gasteropodes proso- 

 branches : Ann. Sc. N'at. ZooL (7) ix. (1890), pp. 89-404. 



1 Science^ iv. p. 50. 



2 P. Fischer, Journ. de Conchyl. (3) xxvii. p. 20L 



