LOWER HELDERBERG ROCKS. iri 



Ortliis perelegaiis ( n. s.). 



Plate XIII. Fia. 4 - 12. 



Shell transversely oval : valves nearly equally convex. Dorsal valve 

 subventricose, more or less depressed along the middle from near the 

 beak to the front : beak small, little elevated above the hinge line. 

 Ventral valve elevated along the middle from the beak towards the 

 front, and sloping laterally : beak small, pointed, incurved, extending 

 beyond that of the opposite valve. Cardinal margin generally sloping 

 a little from the beaks, and rounding imperceptibly into the lateral 

 margins. Area narrow, nearly half as long as the width of the shell. 

 Foramen broad triangular, extending nearly to the apex of the beak. 



Surface marked by fine irregular bifurcating longitudinal striae, crossed 

 by concentric lines of growth. 



This species is chiefly distinguished from the last by its more ventricose dorsal 

 valve, and by the hinge line sloping more from the beaks laterally, giving a gently 

 curved instead of straight outline. The ventral valve, although strongly elevated in 

 the centre towards the beak, is not subcarinate, as in the last; and this gibbosity 

 is lost before reaching the base. Along the middle of the dorsal valve there is a 

 broad undefined depression, which is more conspicuous in the upper part, from the 

 beak half way to the base; below which point, it becomes scarcely marked as a 

 distinguishing character of the shell. This feature, together with the convexity of 

 the valves, contrasts strongly with the nearly flat valve and narrow mesial depression 

 of 0. subcarinata. The striae of this species are often fasciculate; the stronger ones 

 separated by three, four, five or six smaller ones, a feature scarcely perceptible in 

 the other species. 



The internal difi"erences are still more conspicuous. The cavity beneath the beak 

 of the ventral valve is longer than in the preceding, and less angular, and the 

 central ridge more elevated; the whole merging gently into the shell below, without 

 the abrupt and angular termination of 0. subcarinata. This character is well shown 

 both in the shell and in the cast ( fig. 9 & 11). 



The areas of muscular attachment in the dorsal valve are limited by a strong 

 ridge continuous with the brachial processes, which, curving outwards and again 

 inwards, enclose an oval space of varying proportions, through the centre of which 

 extends a longitudinal ridge which divides and ramifies below the muscular area. 



