THE MOLLUSCS OP THE GREAT AFRICAN LAKES. 175 



and if we look into the matter a little more closely, we see 

 that the only real difference between the single outer lateral 

 tooth of the Viviperas and that of the Cyclostomes lies in the 

 fact that the former is somewhat more elongated vertically, 

 and a little less numerously serrated on the crown. 



From this form of Tsenioglossate radula we may pass, by 

 insensible gradations, to what I may call the Cerithio-planaxoid 

 radulas, in which the outer lateral has become still more 

 specialised and perpendicularly lengthened out. Thus the 

 Cerithio-planaxoid radula comes very close to the quasi-Rhipi- 

 doglossate radula of the truly Archi-teenioglossate forms, and, 

 as we should expect from the abundant geological evidence 

 respecting the antiquity of the Cerithidse, what I have termed 

 the Cerithio-planaxoid radula would rank among the more 

 primitive of the existing Tsenioglossate types. It is this con- 

 clusion which I wish to emphasise, for it has a most direct 

 bearing on the nature and affinities of Spekia zonata, as 

 1 shall now proceed to show. 



It has been seen in the descriptive part of this paper that 

 S. zonata has the anatomical peculiarities, and especially the 

 nerves, of several Naticoids, but, unlike any Naticoid hitherto 

 examined, it presents us with a well-developed Cerithio-plan- 

 axoid tongue. The radula of the more typical Naticoids is very 

 marked in type, and more generally resembles the. radulae of 

 the Xenophoridse, the Chenopodidse, and their associates, than it 

 does that of the Cerithio-planaxoid forms. On the other hand, 

 it will at once be admitted by anyone acquainted with such 

 structures, that among these more highly specialised forms 

 there exist numerous individual examples which exhibit more 

 or less distinct traces of the retention of the Cerithio-planaxoid 

 teeth. The radulse of Vermetus, Crucibulum, and Crepi- 

 dula all show undoubted stages in the gradual transformation 

 of the Ceritho-planaxoid radula into that common to the more 

 aberrant types to which these genera respectively belong. 



Now, unquestionably, S. zonata is extremely old. It lives 

 in association with forms which have every appearance of being 

 identical with several Jurassic molluscan types. Therefore 



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