OF THE HALOLIMNIC FAUNA OF LAKE TANGANYIKA. 381 



in the Callovian of Yorkshire. It is also represented in the Callovian of 

 Montreuil Bellay, where a gasteropod fauna, greatly resembling that of 

 the Inferior Oolite of the Anglo-Norman basin, is found to occur. 



8. Bythoceras, Moore. 

 See pp. 238 and 242. There are two species figured, but, so far as I 

 know, no special comparison with Jurassic forms is invited. 



9. Tanganyicia, Cross. 

 See p. 246, T. rufofilosa. In this case also, no special comparison with 

 Jurassic forms is instituted, but its general resemblance to Natica is 

 pointed out. The fine spiral coloured lines are characteristic of this very 

 pretty little shell, which though naticoid in its outline is certainly 

 different as regards shell-svibstance to the regulation Natica. It is said 

 to be a littoral form and occurs in abundance. 



10. Spekia zonata, Cross or Smith, roith the Jurassic genus, Neridoinii,s 



M. and L. 



See pio. 256 and 351. On p. 257, Moore remarks on its naticoid 

 appearance, and considers it so " completely similar to that of numerous 

 fossil naticoid forms that, had it appeared fossilized instead of having 

 been found living in a great fresh-water lake, there is not the slightest 

 •doubt that it would have been placed in one of the numerous fossil 

 genera which are supposed to group themselves about the living Naticas." 

 Yet on p. 349 (the figures are on p. 351) he says : "Again we find that 

 the shells of the Tanganyika genus, Spekia, are practically indistinguish- 

 able from the fossil remains of the marine Jurassic genus, Neridomus.''^ 



In this latter conclusion he is partly correct, for there is no doubt that 

 the affinities of Spekia are with the Nerites rather than with the Naticas. 

 Spekia is neHtoid, not naticoid, but I fail to trace any especial 

 resemblance to Neridomus. If the reader turns to the illustrations on 

 p. 351, he will perceive that the two representations of Spekia are back 

 views, whereas the two intended to represent Neridomus are front views, 

 nor does the author assist the comparison in the text. But if we take a 

 typical Jurassic Neridomus such as N. Hemisphei-ica from the Great 

 ■Oolite of Minchinhampton, which may be regarded as the type on which 

 Neridomus was founded, we at once find that the columellar region is 

 •convex and the shell imperforate, whereas in Spekia zonata the columellar 

 region is extremely concave, and in some sjDecimens a peculiar umbilical 

 slit is noticeable. Hence, beyond the fact that both Spekia and 

 Neridomus belong to the Neritidse there is very little resemblance so far 

 as the anterior aspect is concerned. It may be remarked, in conclusion 

 that Spekia zonata is related to Neritina rather than to Nerita. There is 

 nothing naticoid about it, and moreover its thick epidermis and general 

 aspect are highly suggestive of fresh-water conditio7is, although its shape 

 may be somewhat unusual. 



