180 J. E. S. MOORE. 



needs further working out. In the meantime, however, it 

 could certainly be regarded as a representative of that ancient 

 Littorino-planaxoid stock from which a portion of the Melanio- 

 planaxoids appear to have originated. This view appears to 

 me to receive a certain amount of support from the fact that 

 T. rufofilosa possesses a reproductive apparatus fully deve- 

 loped, which is at present only known in its complete form in 

 some of the Littorino-melanoids ; for this would appear to indi- 

 cate that both rufofilosa and some of the Littorino-melanoids 

 still possess characters which were once common to their 

 ancestral Littorinoid stock. Unquestionably the possession of 

 the complete reproductive apparatus present in rufofilosa 

 stamps a mollusc as an ancient form, for in this feature of its 

 organisation we have seen (p. 160) that it harks back to a time 

 and to those types which were in existence before the Opistho- 

 branchs had become separated from the primitive Tsenio- 

 glossate stock. I conceive, therefore, that in T. rufofilosa 

 we have before us a surviving example of a form that is closely 

 akin to that ancient Littorina with a Planaxoid radula, out of 

 which the modern Littorinas and the modern Planaxidse with 

 their respective fresh-water and terrestrial derivatives have 

 individually sprung. To illustrate the relationships of these 

 forms, we may refer to the diagram given on Plate 19. 



To recapitulate : we have seen that there are indications that 

 the Melaniidse are certainly capable of being split into at least 

 two groups which have no proximate relation to each other. 

 But there is yet a third type of Melania represented by the 

 genus Melanopsis, and this third group is in many ways 

 more interesting than either of the other two. As Bouvier 

 showed, there is very little in common between the zygoneurous 

 nervous system of the genus Melanopsis and that of any of 

 the other forms associated together at present in the family of 

 the Melaniidse. Nor yet does this nervous system approximate 

 to that of any of the marine Ceriths or their derivatives that we 

 know. On the other hand, there is a certain amount of resem- 

 blance both in the nerves and radula of the genus Melanopsis 

 to the similar parts of the Nassopsis of Tanganyika, lu 



