56 JURASSIC BIVALVIA AND GASTROPODA 



arranged in pairs. On the least eroded parts of the surface the ribs and their inter- 

 vals are seen to be crossed by concentric lamellae. The upper margin of the anterior 

 auricle slopes upward from the umbo. Growth stages of the inner margin of the 

 subauricular notch are marked by a series of lamellae. 



Remarks. Staesche has placed Pecten etiveyensis de Loriol (1904, pi. 24, fig. 1) 

 in the synonymy of C. subtextoria, but Mile Dechaseaux has regarded the two forms 

 as distinct, stating that etiveyensis differs in the regularity, equality, and rounded 

 (rather than angular) cross-section of its numerous ribs, points of distinction also 

 emphasized by de Loriol when describing the species. By these criteria, Staesche's 

 figured specimen of " subtextoria " would be referable to etiveyensis. Mile De- 

 chaseaux's figures (pi. 3, figs. 3, 4) of specimens referred to etiveyensis rather contra- 

 dict this distinction, however, as they indicate a decidedly irregular arrangement of 

 the costae, which also appear to have broader intervals than in the typical etiveyensis. 

 In its tall, trigonal form and acute umbonal angle the East African specimen now 

 recorded more closely resembles the specimens which Mile Dechaseaux figures as 

 etiveyensis than the one attributed to subtextoria, although the obtusely angular ribs 

 are like those of the last specimen. It is possible that Staesche's broader conception 

 of the species subtextoria is justified. The references given in the above synonymy 

 are, however, to illustrations of specimens in which the ribbing is less regular and the 

 intervals are broader than in the typical etiveyensis. Pecten bipartitus Futterer (1894 : 

 32, pi. 5, figs. 4, 4a), from Oxfordian beds at Mkusi, 16 miles N.E. of Mtaru, Tangan- 

 yika, does not seem to differ from C. subtextoria ; whereas it was described as having 

 18-20 ribs, 30 can be counted in the figure. 



Chlamys matapwaensis sp. nov. 

 PI. 7, figs. la, b, 2a, b 



Diagnosis. Small, subequivalve, of slight convexity, inequilateral, height (c. 11 

 mm. in larger specimen) just exceeding length (10 mm.). Valves ornamented with 

 22 or rather more slightly unevenly spaced, rounded, smooth riblets of moderate 

 prominence, the outer ones curving outwards towards the adjacent margin ; the 

 riblets may increase in number to a small extent during growth by dichotomy or by 

 intercalation, the latter occurring mainly at a late growth-stage and only in some of 

 the outer intervals. Intervals flat, their average width about the same as that of the 

 riblets, ornamented with very fine concentric threads. Right posterior auricle 

 obtusely triangular ; other auricles unknown complete ; byssal sinus unknown ; 

 part of right anterior auricle closest to body of shell bearing series of closely spaced, 

 equal threads perpendicular to the hinge-margin. 



Holotype and paratype. Nos. LL.35096, LL.35097 respectively (ex B.P. Coll.). 



Locality and horizon. N. of Matapwa, Pindiro area, Tanganyika ; Upper 

 Kimmeridgian. 



Remarks. In Chlamys curvivarians (Dietrich), recorded above, the riblets are 

 narrower and more numerous. 



