178 J. E. S. MOORE. 



already shown that this apparatus is undoubtedly to be regarded 

 as indicatve of an archaic character in those forms which possess 

 it, and the absence of the complete structure in these Melanias 

 is therefore no indication that both they and the genus Tan- 

 ganyikia are not direct descendants of the same approximate 

 phylogenetic stock. 



If, however, we were to assume from these considerations that 

 T. rufofilosa and the amarula type of Melania are repre- 

 sentative of the whole heterogeneous assemblage of organisms 

 which are at present regarded by the conchologists as belong- 

 ing to the single family of the Melaniadse, we shall be speedily 

 disillusioned. In PI. 15, fig. 3, I have represented the nervous 

 system of Melania episcopalis, and in fig. 4 that of 

 Melania aspirata, while in fig. 10 the similar nervous 

 system of Faunus is shown in the relation to the gills and the 

 surrounding parts. All these nervous systems are obviously 

 constructed on a similar plan, and they are all completely 

 different from that type of nervous system which M. amarula 

 and T. rufofilosa both possess. 



But not only do these types of so-called Melanias differ in 

 the character of their nerves ; both series are easily distin- 

 guished by other anatomical features which they possess. In 

 M. episcopalis, M. aspirata, and Faunus, the radular 

 dentition, as represented in PL 15, fig. 14, is obviously of that 

 Littorinoid character which is so marked in the numerous 

 American forms, such as lo, and all the representatives of the 

 genus Pachychilus. 



The conclusions to which the extensive researches of Bouvier 

 into the character of the nervous systems of the Cerithidae and 

 the Melaniidse led him were briefly these: that the simpler 

 dvaloneurous Ceriths form the marine starting-point for what 

 may be called the Cerithio-melanias on the one hand, and for 

 such forms as the Potamids and the genus Tympanotomus on 

 the other. Secondly, that the Planaxids had nothing to do 

 with the Ceriths, but were to be referred to a Littorinoid 

 ancestry. This view of the matter, I believe, is in the main 

 correct, but it does not account for the characters which a 



