THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 199 



Strangely enough, it appears to have lived on, not alone, but 

 in company with several other usually extinct marine Jurassic 

 forms. 



Turning noAv to the characters of the new genus Bytho- 

 ceras, we find no such peculiarities as those I have just 

 described as distinctive of Nassopsis nassa. But never- 

 theless this genus is not without much interest from a mor- 

 phologist's point of view. Nowhere, so far as I am aware, 

 have we a better instance of the fact that shell structure as a 

 means of classification is a " delusion and a snare ;" for in the 

 case of Bythoceras it is not only that the shell in no way 

 foreshadows the animal inside it, but that its surprising simi- 

 larity to Nassopsis is absolutely misleading, and, were we 

 ignorant of the animals contained in both, would lead to a 

 profound error in the close conchological association of the 

 two genera. 



Who would have dreamed, when contemplating the heavy, 

 thick, and highly ornamental shell of Bythoceras Howesii, 

 that the animal it contained bore any close resemblance to 

 that enclosed in the strangely different shell of the genus 

 Tympanotamus ? Yet in the most peculiar features of 

 its radula, nerves, gills, and viscera it strongly resembles 

 Cerithium, Tympanotamus, and their close allies. 



The reproductive organs of Bythoceras are simple and 

 peculiar, and in many other ways the structure of this organism 

 will afford ample material for conjecture concerning the final 

 identification of the really primitive cerithoid type. For 

 which is more ancient, Bythoceras or Cerithium? 1 do 

 not feel competent to aiiswer this question, and I would refer 

 the reader to the broader discussion of this subject in my longer 

 paper on "The Prosobranchiale MoUusca " (in the hands of 

 the editor of the 'Quart. Journ. Micr. Sci.'). 



This much, however, aiay be affirmed with certainty, that 

 Bythoceras is a form closely related to our general concep- 

 tion of the genus Cerithium, with some of the minor features 

 in its radular dentition peculiar to and distinctive of the genus 

 Tympanotamus. 



