MOLLUSOA OF INDIA. 33 



the outermost tooth, and I ligure it from a slide prepared by 

 Mr. W, Moss *. It will be seen that, so far as the central and tivo 

 outer teeth are concerned, little difference is to be found between 

 the European representative and the African. But the outer tooth 

 of C. elegans is very instructive. On the inner angle may be seen 

 about four cusps, very different iu form and size from the large 

 regular series that follow, these last being very sharp and regular 

 and minute as compared with them. Moreover, there is a distinct 

 line of division, corresponding to these four or five larger cusps, all 

 the way down the inside margin of the broad tooth ; this margin, 

 like a selvage, being much thicker than the rest of the basal plate. 

 I think we here have a distinct indication of the fusion of two 

 original teeth into one, as is shown so well in the Mauritian 

 unifasciata. 



Dr. r. H. Troschel, in his excellent work ' Das Gehiss der 

 Schnecken,' which contains such a mass of valuable detail, hgures 

 on plate iv. the radula of Cjjdostoiua elegans (fig. 8), and costu- 

 latus {Qg. 9) of Europe; in his enlarged drawing of the species 

 (fig. 10) he has shown the larger cusps on the interior side of the 

 outermost tooth, but the band of marginal thickening is not indi- 

 cated in the plate, but on p. 70, in his description, it is alluded to. 

 Fig. 11 gives a row of teeth of C. ligatus, from the Cape of Good 

 Hope : in this species it is interesting to find the outermost tooth 

 corresponding with those I describe in the Madagascar and what 

 is said to be a Mauritian shell. In C. ligatus the cuspid edge of 

 the last tooth is divided into nearly three equal curved sections, 

 that on the inner side having larger teethlcts than the middle 

 section. In fig. 12 Troschel gives a radula of Leonia mammiUans, 

 also an African genus and species. In this species the outermost tooth 

 shows also three distinct and differently cuspid divisions on curved 

 edges. This structure presents to my mind a compound tooth of 

 what was originally three distinct and separate side teeth in some 

 far distant ancestor. Dr. Troschel describes these outermost teeth 

 in all their details, but their signification he does not allude to. 



The West-Indian species {vide Troschel, from figs. 13-26) of Chon- 

 dropoma have a type of their own, very different from the European 

 genus Cyelostoma, and depart in a greater degree from Oiopoma on 

 the one hand and Tropidophora on the other. 



Dr. p. Fischer, in his ' Manuel de Conchyliogie,' has figured the 

 radula of Choanopoma to illustrate this organ of the Cyclostomidtc ; 

 it would have been more correct had he selected the radula of 

 Cyclostoma elegans, which is the type of the genus Ci/closfoma, and 

 with which all subgeneric comparisons must be made. 



If the character of the radula has the generic weight which has 

 been given to it by some authors, and I consider it trustworthy 

 when combined, as it always should be, with other characters, 



