THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 173 



radulas represented iu the accompanyiug diagram the areas 

 lettered a, b, c, d, are morphologically the same. But if this is 

 so, the comparison can be pushed much further, since it is 

 apparent that there is a very real morphological distinction 

 between certain areas of the dentition, which is quite inde- 

 pendent of the number of denticles which tlie individual areas 

 may contain. 



What is of primary importance, however, is this, that in a 

 very large number of Rhipidoglossa the radula presents a 

 simple division into separate denticular areas, three on each 

 side of a median line, there being precisely the same morpho- 

 logical distinction between the elements in these areas as that 

 which subsists between the molar and premolar teeth of 

 mammals. We know that the Rhipidoglossa stand in the 

 relation of ancestors to the great Tsenioglossal group, and the 

 question naturally presents itself whether the 3, 1, 3 radula of 

 the Tsenioglossa is not to be interpreted as a condensation into 

 seven single denticles of the more numerous Rhipidoglossate 

 teeth ; for in the latter group the denticles are already diffe- 

 rentiated into a similar number of morphologically distinct 

 denticular areas. But although Troschel pointed out that the 

 Tsenioglossate radula in certain forms approaches that of the 

 Rhipidoglossa, he seems for some reason or other to have 

 missed this fundamental similarity, which becomes apparent 

 throughout the radulse of both groups if we consider the 

 denticular areas, as we have every right to do, and not the sepa- 

 rate teeth. In some Pleurotomaridse^ this division into lateral, 

 angular, and median denticular areas is marked, and as this 

 division is present in most Rhipidoglossa we are fully justified 

 in regarding it as a fundamental feature of molluscan tongues. 

 In fact, the presence of such a division in a group like the 

 Rhipidoglossa appears to be as strong, if not stronger evidence 

 for the fundamental character of the arrangement than that 



» Dall., 'Bull. Mus. Comp. Zool. Havard,' vol. xviii, 1889, Pleuroto- 

 maria adansouiana, C. and F., pi. xxxi, fig. 3. It is not so apparent in 

 Bouvier's figures of P. quoyaua, 'Arch. Zool. Ex.,' 3d serie, vol. vi, pi. xii, 

 fie. 1. 



