i2 4 JURASSIC BIVALVIA AND GASTROPODA 



inflated, subequivalve. Umbones broadly rounded, situated at about anterior third 

 of length, and well incurved to the prosogyrous beaks. Lunular region well im- 

 pressed. Antero-dorsal outline slightly concave, sloping steeply to the low, abruptly 

 rounded anterior end of the shell. Ventral margin asymmetrical, moderately con- 

 vex anteriorly, flattened posteriorly except at extreme end, which bends up sharply 

 to the angular posterior extremity of shell. Postero-dorsal outline moderately 

 convex except posteriorly, where the actual dorsal margin emerges from behind the 

 umbonal profile. Posterior margin short, straight, very oblique, forming obtuse 

 angle with postero-dorsal margin. Posterior umbonal ridges well defined, sigmoidal. 

 Ornament consisting of rounded concentric riblets separated by much narrower 

 intervals and regularly spaced in all stages of growth. 



Holotype. No. LL. 35155, ex B.P. Coll. The only specimen. 



Locality and horizon. 6 miles N.W. of Kidugallo, Tanganyika ; Bajocian. 



Remarks. In shape this species, with its gradual posterior taper, bears a very 

 slight resemblance to the German Bajocian species C. involuta Munster (in Goldfuss, 

 1840 : 250, pi. 151, figs. 14a, b), which needs re-describing on the basis of additional 

 material. It differs from Goldfuss's type-specimen (re-illustrated by Kuhn, 1938 : 

 142, pi. 5, fig. y), which is 10 mm. long, in its larger size, its more elongate outline, 

 and its much stronger inflation. 



Corbula asaharbitensis sp. nov. 

 PI. 19, figs. i8a, b 



Diagnosis. Rather small (length of holotype 5-5 mm.), subtrigonal, well elongated, 

 height about three-fifths of length ; inflation moderate ; whether equivalve or not 

 uncertain. Umbones not prominent, just posterior to mid-length, slightly opistho- 

 gyrous ; an umbonal ridge present in each valve, crossing shell to ventral margin 

 with slight posterior inclination. Posterior end of valves, beyond the umbonal ridge, 

 compressed and subrostrate, tapering to a narrow extremity and with slightly con- 

 cave dorsal margin and almost straight ventral margin. Antero-dorsal outline 

 feebly convex, gently sloping ; anterior margin evenly rounded, curved in continuity 

 with the ventral margin, which is strongly convex along the anterior half of its length. 

 Surface of shell anterior to the umbonal ridge ornamented with evenly spaced con- 

 centric ridges crossed by radial threads, forming a reticulate pattern ; posterior end 

 of shell ornamented with radial riblets only, about 8 in number. The ornament is 

 similar on the two valves. 



Holotype and paratypes. Holotype, no. LL. 13230 ; several paratypes. 



Locality and horizon, i mile N. of Asaharbito, N.E. Kenya ; Bathonian [? or 

 Callovian], Asaharbito Beds. 



Remarks. Before the more complete specimens had been extracted from the 

 matrix it was thought that this species might be related to the European Bathonian 

 form Corbula hulliana Morris, but it is much more elongate than that species and also 

 differs in the presence of radial ornament on both valves. It is a little like the Kim- 



