LAKE TANGANYIKA — AN OLD JURASSIC SEA. 317 



Smith's generic diagnosis of Limnotrochus runs thus : — 

 " Shell trochoid, umbilicated, without an epidermis, spirally 

 ribbed; body-whorl keeled round the middle; aperture non- 

 lyrate within ; with the outer lip oblique, the basal margin 

 broadly sinuated, and the columella edge somewhat reflexed. 

 Operculum horny, paucispiral, litterinoid."^ This description 

 would not do for the living Littorinas of our shores, but it 

 covers the two forms, one from Tanganyika and the other 

 from the marine Jurassic beds, just described. 



I would next direct attention to the very obvious con- 

 chological similarity between the so-called Limnotrochus 

 Kirkii (fig. 6) and the marine genus Xenophora (Onustus), 

 a form which has extended in the ocean from the Devonian to 

 the present time. This genus is not, therefore, typical of the 

 Jurassic period, specially those which I have already described, 

 but it forms one more remarkable example of the marine 

 character of the halolimnic forms. I have represented side by 

 side the L. Kirkii (fig. 6) and an example of Onustus 

 (fig. 6 a), a typical Jurassic forra.^ 



I have already stated that the so-called Lithoglyphus 

 (Spekia) zonatus of Tanganyika (fig. 4) is unquestionably, 

 from the characters of its anatomy, a Naticoid ; and in the infe- 

 rior Oolite there are forms which it would be quite legitimate to 

 regard as coming near this genus. To illustrate this fact I 

 have figured the so-called Neridomus (fig. 4<a) of the inferior 

 Oolite, the affinities of which are doubtful in a high degree. 



Lastly, in Tanganyika there exists a remarkable longitudi- 

 nally sulcated shell known as Melania admirabilis (Smith) ; 

 how closely this form corresponds to those remarkable Oolite 

 shells known as Cerithium subscalariforme will be seen 

 on comparing their respective shells. I did not find this species 

 myself in Tanganyika, and as the animal it contains is not 



1 'Proc. Zool. Soc.,' 18S1, p. 285. 



2 It is needless for me to point out that the two forms of Xenophora here 

 figured from Tanganyika and from the Inferior Oolite are not specifically the 

 same. The so-called Limnotrochus Kirkii of Tanganyika being much 

 more like several modern examples of the genus Onustus (Xenophora). 

 The figures only illustrate the general similarity of such shells. 



VOL. 41, PART 2. NEW SERIES. Y 



