104 Annals of the South African Museum. 



forward obliquity ; the length is greater than the height. The nearly 

 terminal umbones are little conspicuous and weakly incurved. The 

 cardinal margin forms an almost straight line and slopes back very 

 gently from the umbo. It forms posteriorly a sub-angular junction 

 with the relatively long, slightly convex siphonal margin. The 

 frontal margin is very slightly convex, more seldom quite straight, 

 in outline, and passes below by a sharp curve or sub-angular junction 

 into the lower border; this is more convex in profile. The inflation 

 of the valves is relatively weak ; the greatest height occurs at about 

 the middle of the valve. A flattened frontal face is well developed 

 in fully grown specimens. There is no definite carinal angle, but the 

 area is well demarcated by the limits of its particular sculpture, which 

 is contrasted with that of the flank. 



The sculpture of the flank in the young shell consists of relatively 

 strong concentric nodular ribs, four of which are already developed 

 when the shell has attained a height of 10 mm. Subsequently formed 

 ribs increase rapidly in prominence and robustness and become 

 steeply inclined, so that after about ten ribs are developed all reach 

 the inferior margin. The ribs of the adult shell consist of rows of 

 large and prominent tubercles, some of which reach a diameter 

 of 5 mm. Most of the ribs have a forward sweep when traced 

 downwards ; an approximately vertical direction is attained only by 

 those ribs situated in the posterior half of a fully grown valve, and 

 these also are slightly curved in form, with the convex side directed 

 posteriorly. The interspaces are rather narrower than the tubercles 

 of the ribs. In the early and middle adult stages, the tubercles com- 

 posing the ribs are well separated from one another on a given rib. 

 In the late adult and senile stages the tubercles become closely 

 crowded together and are contiguous. Towards the lower border of 

 a fully grown individual the tubercles tend to become elongated in a 

 direction parallel to the shell margin, and they may appear to coalesce 

 with prominent ridges of growth. With senility the ribs become im- 

 perfectly developed, while crowded ridges and furrows of growth are 

 more marked near the pallial margin. In the lower half of the 

 anterior portion of the flank in an adult specimen some tubercles 

 are somewhat unevenly dispersed, giving rise to an appearance of 

 irregularity, with departure from the strictly linear arrangement. 

 In a fully grown specimen about sixteen ribs are developed on the 

 flank. 



The area in the neanic stage is crossed by transverse ribs con- 

 tinuous with those of the flank. In the early and middle adult 

 stages no ribs are developed upon it ; but a line of tubercles limits 



