1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 815 



whether anatomical differences of an important character do not 

 indicate the distinction of Neritina jiuviatilis — and possibly of 

 •other nearly allied Eui'opean species from the ti'opical foims 

 usually classed in the same genus. It may be pointed out here 

 that the conchological characters relied upon in the deter- 

 mination of the two genera are confessedly somewhat obscure. 

 In general, Xerita has a thicker and more solid shell, usually 

 •ornamented with spiral ribs, but these may be absent, as in 

 polita, morio,2)icea, &c. ; when ribs are present they usually project 

 slightly beyond the outer lip, which is then dentate, but it may 

 be smooth. The inner surface of the outer lip is generally 

 dentate, but this character may be absent. The operculum is 

 usually solid, with a granular outei- surface, or with a mai-ginal 

 ^one, and the siiape of the apophyses springing from its inner 

 side is claimed to be characteristic. 



Dr. Ed. von Martens (27), the leading authority on the 

 ■classiiication of the ISTeritacea, maintains that the denticulation 

 -on the inner side of the outer lip and the characteristic sculpture 

 •of the operculum are the most constant differences between 

 Xerita and Xeritina, " in den meisten Fallen auch der allgemeine 

 ■die Meerbewohnerin verkiindende Habitus, der aber bei den 

 kleineren schwarzen Arten weniger hervortritt." In Xpritina 

 the shell is visually thinner and less solid, not ornamented with 

 spiral i"ibs, though these are ]3resent in X. cornea Linn, and 

 especially in its variety suhsulcata, and spirally arranged rows of 

 spines or nodules are not uncommon. The outer lip is smooth 

 and not dentate (it is distinctly crenulate in X. granosa tSowerb. 

 and X. aculeata Gmelin) and the inner denticulations of the 

 outer lip are wanting. The operculum is not granular or sculp- 

 tured, but minute granulations may be discerned with the aid of 

 a lens in several species ; I found them specially well marked in a 

 ■specimen of X. recluziana Guillou. As for the apophyses, after 

 making a careful comparison of these structures in all the species 

 of both genera that I possess, I have concluded that they ofier so 

 many examples of convergence that they are quite unreliable for 

 the purpose of generic distinction, but, as I make no pretension to 

 ■skill as a systematist, my judgment in this matter must be taken 

 for what it is worth. 



A certain number of Neritinse have the last wdiorl of the shell 

 broadly expanded, the aperture enlarged, and the spire reduced, 

 so that they acquire a secondary symjnetiy ; such, for instance, 

 are X. dilatata Brod. and X. crepidularia Lam. It is interesting 

 to note that those forms which tend towards a bilateral symmetry, 

 such as the two species quoted, and also X. auriculata Lam., 

 X. tahitensis Lesson, iV'. hicanaliculata Recluz, are all from the 

 Indian or Pacific Ocean, and are clearly intermediate between the 

 more common spirally coiled ISTeritinse and the genus Septaria 

 ( = Xavicella Lamarck), which is confined to the same regions. 

 On the other hand, the expanded American species of Xeritina, 

 e. g. latissima Brod. and intermedia Sowerby, retain the distinct 

 Proc. Zool. Soc— 1908, No. LII. 52 



