The Invertebrate Fauna of the Uitenhage Series. 159 



straight upper margin and little-curved lower margin. P. baini 

 differs from this in being more equilateral and less produced and 

 truncated posteriorly. 



As regards the generic position of this shell, although the hinge- 

 characters have not been described and I have been unable to inves- 

 tigate them, there is no reason to doubt that we are dealing with a 

 typical Pleuromya, to which genus belong also, in all probability, the 

 majority of the Neocomian forms to which the name Panop&a was 

 formerly applied. There is no reason to suppose that the hinge- 

 characters of P. baini differ essentially from those of P. lutraria, 

 with which it is associated ; and although it was the nature of the 

 hinge that led Krauss to propose for the latter form the separate 

 generic name Anoplomya, it appears plain from Terquem's * detailed 

 studies of the genus Pleuromya that the name proposed by Krauss 

 must be regarded as a synonym a view already adopted by Zittel.f 



GENUS GONIOMYA J. L. K. Agassiz. 



GONIOMYA Sp. 



This genus is represented by a single specimen of a small right 

 valve. It is unfortunately imperfect, with a large part of the shell 

 substance removed, and it is embedded in a very hard matrix which 

 cannot be removed without further injury to the delicate shell. 



Description. The valve has little convexity and is posteriorly well 

 produced. The umbo is rather pointed and prominent and shows 

 a weak fold of the valve-surface extending for a short distance on 

 its posterior side. The shell- substance is very thin and delicate. 

 The ribbed ornaments of the surface are developed already close 

 to the umbonal apex, where the anterior and posterior ribs are very 

 delicate and closely spaced and are steeply inclined to form the 

 V-pattern. The angle of the V is very acute, and the successive 

 angles formed by the junction of the ribs of the two series are 

 situated at first just below the umbonal apex, and then below one 

 another on the flank on a slightly oblique line posteriorly inclined, 

 so that the lowest angles of the sculpture are situated more back- 

 wardly than those above. The most backwardly situated ribs, 

 which do not contribute to the angular ornamentation, are pos- 

 teriorly inclined when traced down from their upper terminations. 

 Posteriorly to the umbo there is a broad smooth area devoid of 

 sculpture on the upper part of the valve, but this is not sharply 

 demarcated from the flank. 



* Terquem (1) ; Terquem (2). t Zittel (5), p. 125. 



