LOWER HELDERBERG ROCKS. 251 



Meiista priiiceps (n. s). 



Plate XLIV. Fig. 1-5. 



Shell ovate : sides sloping towards the 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 : beat 

 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, Avhich are crossed by 

 indistinct concentric lines of growth ; the latter often becoming con- 

 spicuous towards the margins. 



Tliis 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 diiference 

 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, & S a, b, e. Ventral, dorsal, profile and front views of young indivi- 

 duals, where^the sinus is scarcely defined beyond the front, and tlierc is ^jo 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 ; but tliey 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. 



