208 Annals of the South African Museum. 



arched in the peripheral area. To judge by the channelling of the 

 inner surface, the involution was relatively slight. Since the height 

 of the cross-section at the posterior end is slightly over 30 mm., the 

 rate of increase of the whorl was not rapid. 



The umbilicus must have been shallow. The umbilical rim is 

 not abruptly defined, though the surface of the flank slopes down 

 steeply to the spiral suture. Primary ribs starting from the suture 

 are radial in direction and are not of equal strength. At the 

 rounded umbilical margin, some of the ribs (main ribs) swell into 

 a rounded node, and a bifurcation of the rib may here take place, 

 giving rise to ribs of unequal strength. Some ribs, however (inter- 

 mediate ribs), do not bear umbilical nodes, and bifurcation at the 

 nodes on the main ribs does not always take place. Some of the 

 ribs arising from the umbilical nodes have a forward inclination as 

 they pass towards the periphery. On the peripheral side of the 

 middle of the flank, but near the middle, a second set of nodes 

 is developed on the main ribs, stronger than the inner series. 

 At these nodes a division of a rib into two or three peripheral ribs 

 takes place, and these have a well-marked forward inclination. The 

 intermediate ribs have a corresponding forward swing as they pass 

 on to the periphery. The peripheral ribs are of approximately equal 

 strength and pass across the periphery, though their development is 

 less pronounced in the central area. This area is flat, and in better 

 preservation might be weakly sulcate. On either side of it the ribs 

 show traces of a swelling. 



The inner surface of the fragment (that is, the concave surface) is 

 deeply channelled, in such manner as to show that the previous 

 whorl had a distinctly sulcate periphery, and that the central con- 

 cave area was bounded on either side by a line of nodular swellings 

 in the peripheral ribs. 



Occurrence. Collected at Brentford, Knysna Estuary (151h). 



Remarks. It is unfortunate that only a single fragment of this 

 well-characterised form is available for study. The specimen is so 

 imperfect that its intimate relationships cannot be precisely ascer- 

 tained, and the important evidence of young and early adult growth- 

 stages is wanting. There can scarcely be a doubt, however, that we 

 are dealing with a representative of that group within Hoplites (sensu 

 lato) to which Uhlig has given the generic name Acanthodiscus.''''- 

 Whether this group of trituberculate forms constitutes a generic unit 

 may be open to question, in view of the differences in the characters 

 of the youthful stages shown by some of its members, though 



* Uhlig (5), p. 607. 



