VI MANTLE REFLECTED OVER THE SHELL I 75 



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 flattened. 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 repre- 

 sented by a simple thickening at one end of the plate. In Arion, 

 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 independent granides, or else has entirely dis- 

 appeared. 



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 Galeomma and Scintilla the valves are partially 

 concealed by the reflected mantle lobes, and in a remarkable form 

 recently discovered by Dall ^ {Chlamycloconcha) the shell is com- 

 pletely imbedded in the mantle, which is perforated at the 

 anterior end by an orifice for the mouth, and at the posterior 

 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 un- 

 necessary. It is quite possible, on the analogy of the Gaster- 

 opoda 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 deprived 

 of valves, a greatly thickened mantle performing the part of a 

 shelL- 



The following works wdll be found useful for further study of 

 this portion of the subject : — 



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

 braucheb : Ann. Sc. Nat. Zool. (7) ix. (1890), pp. 89-404. 



^ Science, iv. p. 50. 

 * P. Fischer, Journ. de Conchyl. (3) xxvii. p. 201. 



