188 J. E. S. MOORE. 



another form, which although it presents, when adult, the 

 peculiar horns ahove and below the mouth represented in 

 PI. 21, fig. 3, still bears a most remarkable resemblance to 

 both Paramelania and Nassopsis. For this uew form, 

 and before I was acquainted with more than the shells of any 

 of the three, I proposed the third generic name, By thoceras. 

 Now, without further knowledge, anyone judging from the 

 appearance of the shells of these three different genera would 

 certainly regard them as being in all probability closely related 

 to one another. Therefore, holding the opinion that the con- 

 chological method of determining molluscan relationships is 

 utterly unsound, it is with the greatest satisfaction that I 

 am here enabled, by a detailed account of the anatomy of 

 Nassopsis and By thoceras, to show that even in their 

 wider phylogenetic sense these genera bear no relation to 

 each other. The characters of both, however, are of far greater 

 importance to morphologists than that of affording them mate- 

 rial wherewith to exhibit the futility of attempting to determine 

 the nature of molluscan affinities from empty shells. The 

 anatomical features of Nassopsis are in many ways quite 

 unlike those of any forms hitherto described ; and I shall 

 immediately make it clear that this form presents us with an 

 Archi-tsenioglossa of an entirely new type. This being so, 

 when we consider the unquestionably vast antiquity of the 

 lake in which Nassopsis lives, and the fact that its shell, 

 along with numerous others still living in Tanganyika, is 

 specifically indistinguishable from forms which once abounded 

 in the old Jurassic seas,^ it will be readily accorded by those 

 interested in prosobranchiate morphology that in Nassopsis 

 we are presented with an animal which in the future will 

 probably constitute one of the most important prosobranchiate 

 archetypes of which we are in search. 



> " On the Hypothesis that Lake Tanganyika represents an old Jurassic 

 Sea," 'Quart. Journ. Micr. Sci.,' vol. 41, 1898, p. 303. 



