100 Annals of tlie South African Museum. 



the tubercles of the ribs. In the largest individuals, characters of 

 senility are seen in the imperfect rib-formation near the lower 

 margin, and the replacement of sculpture by simple ridges and lines 

 of accretion at the frontal margin, where the ribs may then be seen 

 to terminate at some little distance from the actual margin. 



For a distance of about 15 mm. from the umbonal apex a very 

 delicately nodose, narrow marginal carina is present. It then 

 dwindles abruptly and disappears, while the carinal angle becomes 

 less marked when traced posteriorly and is replaced ultimately by a 

 gentle and rounded fold of the valve surface. 



The area is very narrow anteriorly, and gradually broadens 

 posteriorly. For a distance of about 15 mm. from the umbonal 

 apex it is ornamented by delicate, slightly granular, transverse ridges 

 separated by narrow grooves. The terminations of these ridges at 

 the carinal angle do not correspond strictly with the upper termina- 

 tions of the ribs on the flank, but the ridges on the area are rather 

 more numerous than these. Each of the ridges terminates inwardly 

 in a delicate, transversely elongated node, a row of which takes the 

 place of an inner carina. At a distance greater than 15 mm. from 

 the umbonal apex the area is without sculpture, though two or three 

 further inner-carinal nodes may be present. The area is divided by 

 a well-marked longitudinal groove into a narrower, superior, and 

 broader inferior portion. The groove is continued right to the 

 posterior margin. At the posterior end of a specimen measuring 

 84 mm. in length, the area is about 20 mm. in breadth. The 

 escutcheon is of great size and length, and at the middle of the shell 

 it is much wider than the area. In its anterior half it is of well- 

 excavated, concave form. Its ornaments consist of numerous, fine, 

 beaded ridges or lines of delicate nodes passing from the inner- 

 carinal nodes rather obliquely backwards across to the cardinal 

 margin. At 30 mm. from the umbonal apex these ornamenting lines 

 cease to be formed, and several of them situated posteriorly in the 

 series do not extend to the cardinal margin. Posteriorly, the inner 

 carinal ridge of the valve becomes obsolete and the smooth 

 escutcheon ill-defined. The ligament space is relatively long, 

 measuring 18 mm. in length in a specimen having a length of 

 84 mm. In each valve it shows at the cardinal margin a straight 

 lath-like ridge, becoming gradually thinner when traced back from 

 the umbo, separated from the escutcheon by a straight, deep, narrow 

 groove. 



The central tooth of the left valve is of relatively great size and 

 prominence, and its inferior indentation is deep. The inner surface 



