FROM TANGANYIKA AND KENYA 131 



Family PLEUROMYIDAE Dall 1900 

 Genus PLEUROMYA Agassiz 1845 



Pleuromya didimtuensis sp. now 



PL 19, fig. 16 



Diagnosis. Of small-medium size (length of largest specimen c. 24 mm.), sub- 

 ovate, somewhat projecting postero-dorsally and with subrostrate anterior end ; 

 inequilateral, height about two-thirds of length, beaks at anterior third of length ; 

 shell well inflated mesially, but somewhat compressed postero-dorsally. Umbones 

 broadly rounded, well incurved to the beaks, which are moderately prosogyrous. 

 Postero-dorsal margin feebly convex, subhorizontal, forming a slightly acute angle 

 with the backward-sloping posterior margin ; ventral margin strongly convex 

 posteriorly, where it joins the posterior margin, flattened and slightly convergent 

 with the postero-dorsal outline anteriorly ; antero-dorsal outline feebly concave, 

 steeply sloping ; anterior margin low, strongly convex. Ornament consisting of 

 narrow, rounded concentric ribs, a little sinuous and irregular in places. Internal 

 characters not observable. 



Holotype and paratypes. Nos. LL.35079 and LL.35080-81 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. Owing to erosion the specimens do not clearly show the projecting 

 postero-dorsal corner of the shell, which is one of its most distinctive characters, but 

 this can be restored by studying the growth-lines. In this feature the present species 

 resembles Pteromya tatei (Richardson & Tutcher) (1916 : 52, pi. 8, figs, ^a-c), from 

 the basal Hettangian of England, but that species is larger and has a more strongly 

 convex ventral margin. 



Pleuromya uniformis (J. Sowerby) 

 PL 20, fig. 6 



1813a. Unto uniformis J. Sowerby : 83, pi. 33, fig. 4. 



1900. Pleuromya tellina Agassiz ; Miiller : 536, pi. 18, figs. 3-5. 



1914&. Pleuromya tellina Agassiz ; Hennig : 168, pi. 14, fig. 5. 



1935a. Pleuromya uniformis (J. Sowerby) ; Arkell : 325, pi. 45, figs. 1-13. 



1948. Pleuromya uniformis (J. Sowerby) ; Cox & Arkell : 40. 



Material. Several specimens. 



Localities and horizons. Scarp face, eastern margin of Makoko plain, Baga- 

 moyo hinterland, Tanganyika ; Oxfordian. Kinjele, 5 miles W. of Mtapaia, N. of 

 Tendaguru, Tanganyika ; Upper Kimmeridgian, Indogrammatodon bed. 



