FROM TANGANYIKA AND KENYA 121 



to median ; inequivalve, right valve larger but only slightly more inflated than left ; 

 most inflated part of each valve lying anterior to umbones. Right umbo rather 

 broadly rounded, prominent, higher than more narrowly rounded umbo of left valve ; 

 both are slightly incurved to the feebly prosogyrous beaks. Umbonal outline in 

 right valve rising slightly above the straight, gently inclined postero-dorsal margin, 

 which forms an acute angle with the slightly oblique posterior margin ; the latter 

 curves round below to merge with the straight or feebly convex ventral margin. 

 Antero-dorsal outline very feebly concave, steeply sloping ; anterior margin rounded, 

 feebly convex. In right valve a narrow and very shallow sulcus runs in some speci- 

 mens from the umbo to meet the ventral margin near its posterior extremity, while 

 a second narrow sulcus, scarcely perceptible, runs down the posterior slope from 

 behind the umbo to the point where the posterior and ventral margins meet. In 

 left valve a rounded-off anterior beak ridge is present in early growth-stages. There 

 is no impressed lunule in either valve. Both valves are ornamented with fine and 

 irregular concentric threads, and irregular concentric undulations may also be 

 present ; radial ornament lacking. 



Holotype and paratypes. Nos. LL.35073 and LL.35074-75 respectively, three 

 specimens in all. 



Locality and horizon. Didimtu hill, 2 miles S. of Bur Mayo, N.E. Kenya ; 

 Upper Lias, Toarcian, Didimtu Beds. 



Remarks. This large Corbitla does not closely resemble any Jurassic species 

 hitherto described. It is rather like the well-known Eocene species C. gallica 

 Lamarck, differing in its flatter ventral margin and somewhat smaller size. 



Corbula mandawaensis sp. nov. 

 PI. 19, figs. 7a, b, 8a, b 



Diagnosis. Of medium size for the genus (length of holotype 5-6 mm., of largest 

 specimen c. 9-0 mm.), trigonally ovate, rather strongly inequilateral, carinate but 

 not rostrate posteriorly, height two-thirds of length, beaks at anterior third to 

 quarter of length ; probably very slightly inequivalve (all specimens retaining both 

 valves in position have, however, suffered distortion by compression) ; inflation 

 moderate. In both valves, umbo broadly rounded, well incurved to the rather 

 strongly prosogyrous beak ; umbonal outline rising well above the straight, gently 

 inclined postero-dorsal margin, which forms an obtuse angle with the very oblique, 

 straight posterior margin ; ventral margin evenly and rather feebly convex ; antero- 

 dorsal outline slightly concave, steeply sloping ; anterior margin of moderate con- 

 vexity ; a well-marked, sigmoidal diagonal ridge delimits a flattened or concave 

 posterior area. Ornament consisting of fine, regular concentric threads present on 

 the flank but not on the posterior area. 



Holotype and paratypes. Holotype, no. LL.35148 ; several paratypes, in- 

 cluding nos. LL.35147, LL.35149, but many are imperfect or crushed. All ex B.P. 

 Coll. 



