174 J. E. S. MOORE. 



which may and ought to be obtained by the study of the 

 ontological formation of the teeth. Consider also the very 

 marked existence of this division revealed by the study of the 

 existing figures of the Dochoglossa, while in the Cephalopods 

 themselves we frequently encounter the regular Tsenioglossate 

 type of tongue. 



From these considerations it is clearly incontestable that in 

 a large series of molluscs of very different types there exists a 

 fundamental similarity of plan, or symmetry, upon which the 

 dentition has been formed ; and the conception of this unity 

 of groundwork seems to me important, in that it throws light 

 upon the means whereby the radulse of those groups which 

 have emanated from the Rhipidoglossa, the Tsenioglossa^ the 

 Rachiglossa, the Ptenoglossa, and the Opisthobranchs have 

 respectively been evolved. 



In the light of these general conclusions respecting the unity 

 of nature,andtheprobableinter-relationships subsisting between 

 the different molluscan dentitions, we may pass now with advan- 

 tage to certain more particular considerations, which are perti- 

 nent to the determination of the true phylogeuetic position of 

 the molluscs with which this paper is primarily concerned. 



We have seen that the Rhipidoglossate radula may be made to 

 pass, by a series of gradations, through forms like Trochatella 

 into that of the Tsenioglossa such as cystula; while in the 

 genus Ovula, the teeth in both the lateral and angular areas 

 tend to be split up. If the above conception of the forma- 

 tion of the Tsenioglossate radula from the Rhipidoglossate type 

 be true, we should expect to find some definite approach to 

 the Rhipidoglossate condition in the Archi-tsenioglossa as 

 they at present exist. Through the work of Bouvier and 

 Lacase-Duthier there seems much reason to believe that the 

 Cystostomidse have originated from the ancient Littorinas, a 

 group wiiich, if fossil identifications can be trusted, is as old 

 as any that we know. 



Now there is a whole series of gradations between the quasi- 

 Rhipidoglossate radula of the Cyclostomes and that of the 

 Paludinas, another unquestionably Archi-tsenioglossate group j 



