110 Annals of the South African Museum. 



Confining our attention to the Pseudo-quadratae, it is difficult to 

 believe that the salient characters by which all the members are 

 distinguished could have been called forth by the ordinary process 

 of natural selection alone. The acquirement of costation on the 

 area coincident in direction with lines of accretion ; the encroach- 

 ment of this simple ornament from the area to the transcrescently 

 sculptured flank in the late adult and senile stages ; the prodigal 

 expenditure of shell substance in producing massive sculpture of an 

 increasingly irregular type in these same growth-stages; and the 

 evident variation in many characters which indicates a certain 

 instability of type ; all these point to racial degeneracy. A com- 

 parison of the Pseudo-quadratse from India, South Africa, and South 

 America, shows that in all of them this degeneracy was expressed 

 as the culmination of a definite sequence of developmental phases. 

 The facts do not warrant the supposition that the members of 

 the group were wholly independent of one another. Their simi- 

 larity in leading characters was perhaps due, in varying degree, to 

 a number of causes : a certain community of ancestry ; continued 

 intercourse until checked by divergence ; and at the same time the 

 constant influence of natural selection ; all these may have played 

 a part. But it seems difficult to escape from the belief that innate 

 racial tendencies of a very definite kind found their expression in 

 representatives of these Trigonia-stocks at the same geological 

 period. Had the members of these genetic series been more 

 passive, so to speak ; had they been more plastic and more com- 

 pletely amenable to the call of natural selection, it is scarcely con- 

 ceivable that environmental conditions should not have occurred 

 at some earlier time which would be capable of producing closely 

 similar results. But the advent of the Quadratic in Europe, and of 

 the comparable though independent Pseudo-quadratse of the south, 

 took place, so far' as we know, invariably in Neocomian times. 



TKIGONIA VAU Sharpe. 

 Plate "VI., figs. 1, la, 2, 2a, 26, 3. 



1856. Tru/oii/a ran D. Sharpe, Trans. Geol. Soc. Lond., ser. 2, 



vol. vii., p. 194, pi. xxii., fig. 5. 



? 1867. Triijonia van, E. Tate, Quart. Journ. Geol. Soc., vol. xxiii., 

 pi. vii., fig. 8. 



Occurrence. The record of occurrence given by Sharpe was 

 " Sunday Eiver, in greenish-grey grit, with fragments of wood 

 and shells ; and in a shelly grit at the Zwartkop Eiver." Speci- 



