SMITH : MOLLUSCA OF LAKE TANGANYIKA. 79 



Edgaria will probably prove closely related to Lmigeria, and Leroya 

 may be merely a solid form of Lanistes, as suggested by Dr. von 

 Martens. Most of the remainder are quite small forms, and it will 

 certainly be a very long time before their complete history is known. 

 It therefore becomes mere conjecture to suppose that they bear any 

 relationship with marine forms, either living or extinct, merely 

 because they have a thalassoid appearance. Many fresh- water shells 

 in other parts of the world have this same facies. A good example of 

 this solid marine aspect is met with in the so-called Melania brevis 

 of D'Orbigny, from the rivers of Cuba. A feature common to this 

 species and the Tanganyikan thalassoid shells is the feeble development 

 of the periostracum, it being in some apparently entirely absent, whilst 

 in othei's it is extremely thin or hardly noticeable. In considering 

 the supposed resemblance between some of these Tanganyikan shells 

 and certain Jurassic fossils, I will take the species in the order in 

 which they occur in " The Tanganyika Problem." 



1. Paramelania Damoni and Puepurina bellona. 



There certainly is a very strong resemblance in this case, and 

 I must agree with Mr. Moore that the two forms appear to be 

 generically inseparable, but I cannot endorse the opinion which, 

 according to Mr. Moore's work, I am supposed to have expressed, 

 that "even within a specific range, there is no valid conchological 

 distinction " between them. Slight difiPerences in the general form, 

 in sculpture, and the aperture, preclude such a decision. But this 

 species of Paramelania has also been considered by Mr. C. A. White 

 and Dr. Leopold Tausch as belonging to the fresh-water genus 

 PyrgiiUfera, which occurs in the Bear River Laramie beds of Wyoming 

 and Utah, and also in the Upper Chalk at Ajka in Hungaiy, and 

 I must confess that there is little to distinguish these fossils generically 

 from Paramelania. One species, es])ecmllj Pi/rgulifera Pichleri, displays 

 all the features of the Tanganyikan shell in a remarkable manner. 

 The general form and character of the sculpture is of the same type 

 in both, and the apertures are quite similar, both having continuous 

 peristomes and a peculiar effusion at the anterior end. It is interesting 

 to again call attention to the similarity of these lacustrine forms, 

 because we should rather expect to find a fossil representative of the 

 Tanganyikan shell in these Cretaceous deposits than in the older 

 Jurassic formations. 



2. Nassopsis nassa and Pttrpurina inflata. 



The shell depicted by Moore appears to be the Lavigeria coronata 

 of Bourguignat, and it certainly is not the Melania nassa of Woodward. 

 I should here mention that the genus Nassopsis is a synonym of 

 Lavigeria, which has two years' priority. 



The genus Lavigeria I do not consider the same as Purpurina, for 

 it differs in having a tubercular prominence on the columella, which 

 seems to be entirely absent in the fossil form. 



If we admit that both Paramelania and Nassopsis are congeneric 

 with Purpurina we are placed in a very awkward position. These 



