124 Annals of the South African Museum. 



Another South American form which is probably also nearly 

 related, is Trigonia eximia R. A. Philippi,* from the Tinguirica 

 valley in Chili. This represents a type of shell very closely com- 

 parable with T. conocardiiformis, but the complete differentiation of 

 the ribs into a posterior and an anterior series is a marked feature 

 of the adult stage. With regard to this character of the ribbing, 

 the form described by Burckhardt under the name Trigonia an . 

 conocardiiformis may be regarded as illustrating a somewhat inter- 

 mediate type of sculpture between the two extremes, T. conocardii- 

 formis and T. eximia. Another probably allied shell has been 

 recorded by Haupt f from the Neocomian of Loteno, on the Rio 

 Neuquen, on the east slope of the Argentine Cordillera, under the 

 name Trigonia cf. eximia Philippi. This is said to differ from 

 Philippi's type chiefly in having the dividing line between the 

 anterior and posterior ribbing more obliquely directed. T. eximia 

 is referred by Haupt to the section Undulatae, but this is an obvious 

 error. These aberrant forms in South Africa and South America 

 have in common certain peculiar characters of shape and ornamenta- 

 tion by which they differ in marked manner from all divisions of 

 the genus known to occur in the Jurassic rocks. It is not improbable 

 that we are here dealing with representatives of several parallel 

 series developed from some common ancestral species or group of 

 species, showing rapid departure from the ancestral type along 

 similar lines of development, the successive phases being attained, 

 however, at an unequal rate. On the other hand, the possibility 

 that convergence is illustrated is not remote. In the above descrip- 

 tion of T. conocardiiformis, differences observable in the plan of 

 sculpture after the close of the neanic stage are ascribed to individual 

 variation. I believe this to be sufficient to account for such 

 differences, but the possibility is not excluded that in the Uitenhage 

 beds, two very closely similar forms, undergoing parallel develop- 

 ment, are present. It may be that those individuals which show 

 some division of the ribbing into two distinct sets forming an angle 

 with one another, at the beginning of the adult stage, illustrate a 

 series which has passed through a stage similar to that represented 

 in the adult T. eximia, and that this stage, with angularly disposed 

 costate ornamentation, has become suppressed by tachygenesis and 

 reduction. The entire absence of characters of sculpture which 

 approach those of T. eximia, in the other individuals from Cape 

 Colony, might probably be due to the same cause. 



* E, A. Philippi (1), p. 76, pi. xxxiv., fig. 8. 

 f Haupt (1), p. 216. 



