The Invertebrate Fauna of the Uitenhage Series. 99 



TRIGONIA ROGERS: sp. nov. 

 Plate III., figs. 3, 3a ; IV., fig. 1 ; V., fig. 2. 



Description. The shell is of somewhat elongated form; it is 

 anteriorly high and is posteriorly more slender in outline and con- 

 siderably produced. The valves are moderately inflated in the 

 anterior half of the shell, and are posteriorly more compressed. 

 Anteriorly, the surface of the valve curves round towards the frontal 

 margin so as to give the shell some appearance of anterior flattening 

 or even truncation, most marked in specimens of large size. The 

 umbones are situated at about one-quarter of the total length from 

 the anterior extremity. They are prominent, well incurved, and 

 slightly recurved. The cardinal margin slopes down gradually, 

 forming a long, almost straight outline, and passes by a rather 

 abrupt curve into the short, convex posterior border. In front of 

 the umbo the valve margin falls steeply at once to form the lengthy 

 frontal border, gently and regularly convex in outline, which passes 

 by a regular and broad curve, without break, into the lower border. 

 This in turn has a gently convex outline, and slopes up gradually 

 towards the posterior margin. The greatest height falls at the umbo. 

 The costae of the flank are concentric in arrangement until the 

 close of the neanic stage, becoming then more and more downwardly 

 inclined, when traced from their commencement at the carinal 

 angle. In an adult specimen the ribs in the anterior third of the 

 shell are not at all curved in form as they pass obliquely down the 

 flank, but on the frontal face of the valve they curve rapidly so as to 

 approach the frontal margin horizontally. The successive costae are 

 more and more steeply directed until at about the middle of the 

 valve their direction is vertical. Posteriorly to this they have a very 

 slight backward inclination. The ribs in the anterior, inflated part 

 of the shell are prominent, strongly nodose or tubercular, and are 

 separated by interspaces as broad as the ribs themselves ; on the 

 frontal face the interspaces are much wider. The ribs of the 

 posterior half of the shell are narrower and less prominent in 

 character, are only weakly nodose, and are more closely crowded 

 together. In a large individual, about a dozen large, prominent ribs 

 may be counted in the anterior part of the valve. In the early adult 

 period, occasional short rows of delicate nodes of unequal extent may 

 be intercalated on the surface between the main ribs where these 

 approach the valve margin on the frontal face. On the surface 

 of the valves clearly marked lines of growth are well seen as they 

 cross the interspaces, but are either little conspicuous or absent on 



