LOWER HELDERBERG ROCKS. 
25 s 
Merista princeps (n. s.). 
Plate XLIY. Fig. 1-5. 
Shell ovate : sides sloping towards tlie beaks at an angle of about 60°. 
Ventral valve more or less profoundly arcuate longitudinally, most 
ventricose near the central and umbonial region, depressed below and 
having a broad shallow flat or subangular mesial sinus in front, termi¬ 
nating (in old specimens) in a linguiform extension which is bent up¬ 
wards at right angles to the longitudinal plane of the shell : beak 
strongly incurved. Dorsal valve gibbous and extremely elevated along 
the middle, which becomes in front a strong rounded mesial elevation; 
sides of the valve declining very abruptly to'the baso-lateral margins : 
beak closely incurved beneath that of the opposite valve. 
Surface marked by obscure fine radiating striae, which are crossed by 
indistinct concentric lines of growth; the latter often becoming con¬ 
spicuous towards the margins. 
This large and fine species differs considerably in the form and depth of the me¬ 
sial sinus, as well as in the greater or less extension of the mesial prolongation of 
the anterior border of the ventral valve. In some of the specimens the sinus is very 
shallow and flattened within, while in other instances it is more impressed and 
angular in the middle : other specimens present intermediate grades of difference 
in this respect, the sinus being nearly flat within, and marked by a narrow, nearly 
linear, deeper depression along its middle. Young individuals are almost entirely 
destitute of a sinus, though the front of the dorsal valve in such cases is usually 
slightly elevated. 
This is not a common species ; and it is probable that a larger collection of spe¬ 
cimens would show some gradations of form and exterior characters, which would 
unite more intimately what now appear extreme varieties. 
Fig. 1 a, b ; 2 a, b, & 3 a, b, c. Yentral, dorsal, profile and front views of young indivi¬ 
duals, where the sinus is scarcely defined beyond the front, and there is no me¬ 
sial fold on the dorsal valve. 
These specimens preserve so much the character of Merista Icevis, that I have 
some hesitation in placing them under this species; hut they are more extended 
in front, and the outline less regularly curved. 
Fig. 4 a, b, c, d. Dorsal, ventral, profile and front views, in which the characteristic fea¬ 
tures of the species are fully developed. 
