The Invertebrate Fauna of tJie Uitenhage Series. 157 



Occurrence. Found in the cliff on Buck Kraal, Sunday's Eiver 



Remarks. The doubtful reference of this form to Mactra is, it 

 must be admitted, unsatisfactory. The generic position, in fact, is 

 very uncertain, for the characters of the hinge are unknown and the 

 ligament space is relatively extensive. The general aspect of the 

 shell does not accord well with either Cyprina or Meretrix. The 

 specimens share with Mactra angulata J. de C. Sow. (Blackdown 

 Beds),* the sub-angular junction of the posterior and inferior margins 

 and the presence of a denned ridge running obliquely from the 

 umbo down to the base of the posterior margin. In M. angulata, 

 however, the shell is more triangular and less ovate in outline, and 

 the umbonal region is more prominent and tumid, less anteriorly 

 placed, and less strongly directed forwards. 



Mactra warrenana Meek and Hayden (Cretaceous of Dakota) i is 

 in some degree comparable, but differs by its more trigonal form, 

 more prominent umbonal region, and the presence of a defined 

 lunule of relatively large size. 



GENUS PLEUEOMYA L. Agassiz. 



PLEUROMYA BAINI (Sharpe). 



Plate VIII., figs. 4, 4a. 



1856. Myacites? bainii D. Sharpe, Trans. Geol. Soc. Lond., ser. 2, 

 vol. vii., p. 195, pi. xxii., fig. 7. 



Occurrence. The locality given by Sharpe was merely " Sunday 

 Eiver." Specimens were collected by Messrs. Eogers and Schwarz 

 at Grass Eidge, 3 miles east-north-east of Uitenhage (317, 318, 

 334, 335), from an outcrop of nodular limestone, the highest fossili- 

 ferous outcrop of this locality, where many characteristic fossils of 

 the Sunday's Eiver Beds were found. J P. baini was also found to 

 occur commonly in the railway cutting between milestones 24^-24f 

 on the line from Uitenhage to Graaff-Eeinet. Mr. Eogers obtained 

 a very well-preserved specimen from the highest beds in the kloof 

 behind Colchester, Sunday's Eiver Valley (495g). 



Remarks. There is some variation in the form of the shell, and 

 the inferior margin, though never showing a strongly curved outline, 

 is not always so straight as depicted in Sharpe's figure. When well 



* J. de C. Sowerby (2), p. 341, pi. xvi., fig. 9. 

 f Meek (2), p. 208, pi. xxx., fig. 7. 

 | Eogers and Schwarz (1), p. 9. 



