LOWER HELDERBERG ROCKS. 
171 
Orthis perelegans (n.s.). 
Plate XIII. Fig. 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. 
Yentral 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 strise, 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 strise 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 differences 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. 
