The Invertebrate Fauna of the Uitenhage Series. 147 



at once falls very steeply with very gently convex outline. This 

 passes in turn by a curve (broader than that of the postero-dorsal 

 outline) into the inferior margin, which has an almost straight out- 

 line. The short frontal border has an evenly convex outline. The 

 shell is greatly inflated below the umbonal region, but the valves are 

 compressed in the neighbourhood of the posterior border. 



The outer surface of the very thin shell-wall, so far as it has been 

 observed, appears to show only the faintest traces of radial striae, 

 but delicate radial linear markings are to be seen on the surface of a 

 cast of the interior. The punctate ornaments of the surface may be 

 separated from one another, on the same radial line, by a space of 

 at least 3 mm. in the posterior part of the shell, near the pallial 

 margin. They are less strongly developed and are more closely 

 spaced in the anterior part of the valves. 



The well-marked angular linear impression on the posterior side 

 of the umbonal region of the internal cast, has its apex extending 

 almost to the umbonal apex, so that the apical part of the angular 

 line is hidden from view by the incurvation of the contiguous 

 umbones, in a specimen in which the valves are united. The line 

 forms a very acute angle. The posterior limb is relatively weakly 

 impressed, but the anterior limb is well incised and has a very slight 

 backward inclination when traced away from the umbonal apex. In 

 the specimen here described, it only extends for about 5 mm. and 

 then ceases abruptly, while the posterior limb extends for at least 

 8 mm. and dwindles away. 



Dimensions. 



Length 25 mm. 



Height, measured from the unibo 17 ,, 



Greatest depth of a single valve 9 ,, 



Occurrence. Collected at a kloof, S. 5 W. from Comley's house, 

 on the right bank of Sunday's River (83h). 



Remarks . Only one specimen of this form has been found, but it 

 is well characterised by its long cardinal and pallial margins and 

 the relatively small height in comparison with length, as well as by 

 the peculiar form of the angular linear impression on the cast in the 

 umbonal region. The specimen is preserved in the form of an 

 internal cast with portions of the shell adhering, and it is not in the 

 least distorted. So far as I am aware, there is no described species 

 of Thetironia with which this new form can be brought into close 

 comparison. 



