The Invertebrate Fauna of the Uitenhage Series. 121 



even at this early stage, but the ribs encroach slightly upon the 

 smooth escutcheon. This is clearly seen in the neanic stage, where 

 the escutcheon begins to increase rapidly in breadth. Very close to 

 the umbonal apex, however, it is difficult to observe traces of an 

 escutcheon, and here the attenuated and delicate linear ribs of the 

 area pass obliquely forward until almost reaching the cardinal 

 margin, when they die out. Where they disappear, very close to 

 the cardinal border, the ribs are extremely attenuated and crowded, 

 while directed to form an acute angle with the valve-border. The 

 longitudinal linear groove of the area which becomes so marked a 

 feature in the adult, is only faintly indicated at the close of the 

 neanic stage. 



As the succeeding growth-stage is entered upon that is, when the 

 valve exceeds 8 mm. in height the carinal angle rapidly becomes 

 rounded and blunt, and soon takes the form of a broad fold. The 

 ribs of the flank are now prominent and robust, and are separated 

 by interspaces 3 mm. broad. They pass obliquely downwards when 

 traced forwards. There is some variation shown in the relation of 

 the anterior ribs to the succeeding posterior ones in the adult stage. 

 Sometimes the first four or five strong ribs have their posterior 

 terminations on the flank at successively greater distances from the 

 carinal fold. Immediately behind these, and forming a separate 

 series having an almost vertical arrangement, are the numerous and 

 much more delicate ribs of the posterior series. They have their 

 superior terminations immediately below the carinal fold. The sixth 

 or seventh rib of the anterior series is in continuity with the second 

 or third rib of the posterior series, forming the lower and more 

 strongly developed portion of it, this lower portion being directed to 

 form a very obtuse angle with the upper portion. Posteriorly to 

 this, the ribs are not divisible into two series. In other specimens, 

 however, the successively formed ribs have their upper terminations 

 alike close under the carinal fold of the valve, and there is no 

 tendency to division of the ribs into two series. There is also con- 

 siderable variation in the strength and number of the ribs. In some 

 specimens the ribs are more strongly developed and are rather less 

 numerous than in other individuals of equal size ; sometimes their 

 more strongly swollen character may be particularly noticeable on 

 the posterior half of the flank. 



The area throughout the adult stage is devoid of ornamentation 

 except the well-marked longitudinal groove. The escutcheon is of 

 very elongated lanceolate form. It is not separated from the area 

 by any definite carinal ridge, but is well marked off for the most 



