1908.] ASPIDOBKANCH GASTROPOD MOLLUSCS, 849' 



posterior part of the foot and the visceral mass, and the last- 

 named is interposed between the hinder ends of the elongate 

 and perfectly symmetrical right and left shell-muscles. Of course, 

 the analogy between the Fissurellidfe and Neritidje is incomplete 

 in many respects. The ancestor of the latter family must have 

 suffered great reduction in the organs of the post-torsional right 

 side, such as the ctenidium, before any tendency to detorsion 

 manifested itself. All that I wish to point out here is that, in 

 any discussion of the causes of torsion and detorsion in gastropod 

 molluscs, the action of the muscles should be considered more 

 carefully than has hitherto been the case. I do not pi-opose to 

 pursue the subject further in this place, as it would involve a 

 long and detailed comparison of the regional anatomy of many 

 different gastropods, but before leaving it I must guard myself 

 against the imputation of giving a Lamarckian explanation of a 

 particular course of molluscan evolution. I have suggested a 

 physiological explanation, and this is necessarily Lamarckian in 

 foi-m. The reader can easily translate my suggestion into terms 

 of Natural Selection, and it would be no compliment to his 

 intelligence if I were to occupy an additional page of print in 

 doing so. 



Of other external features, I need only allude to the following.. 

 The snout is so short that it can hardly be said to exist ; there is 

 neither pretentacular nor postentacular elongation in the cephalic 

 . or postcephalic regions in the sense indicated by Amaudrut (1). 

 The opercular lobe is relatively very large and muscular; its 

 outline is semilunar, the left corner deeply indented to receive 

 the apophyses of the operculum (fig. 29). The epipodium is. 

 represented by a low ridge, devoid of any tentacles or outgrowths, 

 extending from behind the tentacles to the posterior end of the 

 operculum on each side. It is somewhat enlarged in the opercular 

 region, but is inconspicuous in contracted specimens, because it is 

 comi:)ressed between the hind end of the foot and the opercular 

 lobe. 



The anterior border of the mantle is muscular, and in marine 

 species [Nerita) more or less frilled. The remainder of the 

 mantle is extremely thin. 



The tentacles are conical, commonly decorated w-ith alternate- 

 stripes of black and white, and are short in all the fresh- 

 water species {Paranerita and Xeritina) that I have examined,, 

 but long and slender in the marine species {Neriia). 



Various accounts have been given of the so-called cephalic jaenis 

 in the Neritidse. It was figured by Quoy and Gaimard (35), 

 referred to in somewhat doubtful tei-ms by Moquin-Tandon (28), 

 overlooked by Claparede (12). Bouvier (9) gives a i-emarkable 

 figure of this organ in Neritina cariosa, and Thiele makes a very 

 short reference to it and gives a small figure of its appearance 

 in transverse section. The organ in question is without doubt a 

 modification of the inner part of the base of the right tentacle 

 and is a good external indication of the male sex. Its position 



54* 



