The Invertebrate Fauna of the Uitenhage Series. 209 



Professor Uhlig has suggested that the occurrence of ccenogenetic 

 phenomena may account for these. So far as the characters of 

 sculpture of this fragment are concerned, a very similar type is seen 

 in the adult stage of Acanthodisciis hystricoides (Uhlig), * from the 

 upper Teschener Schichten of Silesia (correlated by Uhlig with the 

 Valanginian). To judge from a comparison of such scanty material 

 with the figures of A. hystricoides, the African specimen differs in 

 the more nearly circular section of the whorl, in the occurrence 

 of groups of three peripheral ribs arising from the outer tubercles of 

 the flank, and perhaps also in the greater number of ribs inter- 

 mediate between the main ribs. It also shows a more strong 

 forward inclination of the ribs as they pass from the flank to the 

 periphery. 



It is impossible to say with certainty whether this form bears any 

 close relationship to Tate's Ammonites subanceps, which also came 

 from the Uitenhage beds. Tate's figured specimen f is the only one 

 of its kind known from these beds, and it is a small individual, 

 representing in all probability an immature stage of growth. This 

 probability, and the great disparity in size between Tate's type and 

 the fragment of an adult Acanthodiscus above described, makes a 

 comparison difficult and unsatisfactory. The fragment here dealt 

 with formed part of an individual exceeding 110 mm. in diameter, 

 with an umbilical diameter, measured from the inner tubercles, of 

 probably 45 mm. Tate's specimen measures less than 20 mm. in 

 greatest diameter. Allowing for this, however, there is some simi- 

 larity in the type of sculpture in the two forms. If they are closely 

 related the points of distinction that exist may well be due to the 

 fact that different stages of growth are brought into comparison. In 

 the specimen from Knysna the sulcation of the peripheral area is 

 very weakly marked, but it is seen from a comparison of the outer 

 and inner sides of the whorl that this is a modification accompanying 

 advancing growth, and that the previous whorl was much more 

 sulcate. It is also probable that the reduction of this character 

 is to some extent only apparent, owing to the removal of the 

 shell-substance on the outer side by weathering. Other points in 

 which the specimen differs from Tate's type are the presence of 

 well-marked nodes at the umbilical margin, the situation of the 

 second series of nodes a little nearer to the periphery, the stronger 

 forward inclination of the peripheral ribs, and the rather broader 

 periphery and more nearly circular outline of the whorl in cross- 



* Uhlig (2), p. 39, Taf. i., fig. 8. 

 f Tate (1), p. 150, pi. vii., fig. 3. 



