198 J. E. S. MOORE. 



with a feature which, like the caecum, connects Nassopsis 

 on the one hand with the Rhipidoglossa, and on the other 

 with the Archi-paludines. In like manner the wide separation 

 of the cerebral ganglia from one another, and the comparatively 

 great length of the cerebro-pleural connectives, are certainly 

 features which recall the primitive scattered condition of these 

 parts. In fact, the position of the pleural ganglia in Nassopsis 

 represents a condition of development which is really halfway 

 between what I may term the hypo- and the epi-athroid 

 types, the nervous system of Nassopsis marking, in fact, 

 a third, intermediate, or dystenoid type. 



Again, in the scalariform pedal cords with which Nas- 

 sopsis is provided we have another most important feature, 

 showing that this form is not far from the border-land between 

 the older groups of Teenioglossa and their Rhipidoglossate 

 ancestry. 



Nassopsis in many ways is more primitive than Paludina, 

 but at the same time, as will have been seen, it bears no very 

 approximate relation to this form ; neither is it very near the 

 ancient group of Littorinas, nor indeed to any of the indi- 

 viduals which at present are regarded as constituting the 

 archi-tsenioglossate group. 



The morphological interest of Nassopsis lies, therefore, in 

 this, that it presents us with a new starting-point from whence to 

 study the inter-relationships of the great Prosobranchiate 

 order.^ 



Nassopsis, therefore, presents us with a type of organisation 

 which there are conchological reasons for believing is similar 

 to that possessed by several species of a genus which was 

 once abundant in the sea, but which has long since become 

 extinct outside the confines of Lake Tanganyika, where, 



' I shall elsewhere emphasise more fully the importance of Nassopsis 

 as an archetype, and it is therefore needless for me here to do more than point 

 out the fact that the above conclusions respecting the great morphological 

 antiquity of this form are fully substantiated by the very unexpected simi- 

 huity which three of the rather marked varieties of the shells of Nassopsis 

 present to the genus Purpurina from the old Jurassic seas. 



