146 JURASSIC BIVALVIA AND GASTROPODA 



Holotype. No. GG.10313 ; there is no other material except an associated 

 internal mould, which scarcely ranks as a paratype. 



Locality and horizon. Mpilipili stream at a point about 1 mile N.E. of Mitole, 

 Tanganyika ; Upper Kimmeridgian. 



Remarks. This form resembles the type-species of Chartronella, C. digoniata 

 (Cossmann) (1902 : 199, pi. 4, figs. 24-26), from the Hettangian of France, in its 

 bicarinate but otherwise almost smooth whorls, in its convex base, and in the absence 

 of an umbilicus, but it differs in the fact that the upper of its two carinae marks the 

 edge of a subhorizontal ledge instead of a steeply sloping sutural ramp, so that its 

 whorls are much lower than in Cossmann's species. 



Family CIRRIDAE Cossmann 1916 



Genus CIRRUS J. Sowerby 1815 



Cirrus mazer asensis sp. nov. 

 PL 24, figs. 8a, b 



Diagnosis. Of medium size (original height of holotype, allowing for missing 

 last whorl, c. 25 mm. ; height of spire as preserved, c. 15 mm.), sinistral. Spire 

 coeloconoid with a very acute apex, and consisting of whorls which are at first almost 

 flat but become increasingly convex during growth. Ornament consisting of vari- 

 ably but mostly strongly prosocline transverse riblets and of strong concentric 

 threads which override them and occupy their intervals ; the ribs, which appear 

 close to the upper suture but die out before reaching the lower one, are separated by 

 intervals which are almost equal to them in width on the earlier whorls but become 

 almost twice as wide on the last preserved whorl ; the spirals number 6 on the later 

 whorls and are separated by intervals about twice as wide. Last whorl and aperture 

 unknown. 



Holotype. No. GG.6524, an external mould of the spire of the shell from which 

 squeezes have been prepared. 



Locality and horizon. Ribe, 9 miles N.E. of Mazeras, Kenya ; Bajocian (?), 

 Mazeras Sandstones. 



Remarks. The strongly coeloconoid spire suggests that the last whorl (no longer 

 preserved) was umbilicate, so that the species is a Cirrus rather than a Hamusina. 

 No species with identical ornament can be traced in the literature. 



Genus HAMUSINA Gemmellaro 1878 



Hamusina thompsoni sp. nov. 

 PL 24, figs. 9«, b 



Specific name. After Mr. A. O. Thompson, of the Geological Survey of Kenya. 



Diagnosis. Shell small for the genus (height 11 mm.), acute, sinistral, with 



aperture occupying about one-third of total height ; spire angle c. 30 °. Periphery 



