The Invertebrate Fauna of the Uitenhage Scries. 97 



Occurrence. Messrs. Eogers and Schwarz record the occurrence 

 of this shell ("the large variety of T. ventricosa") * in the railway 

 cutting between milestones 24^ 24f on the line from Uitenhage to 

 Graaff-Keinet, about three miles from Uitenhage. The specimen 

 from the South African Museum, here figured, is probably from the 

 Sunday's River. Krauss met with this form in the left bank of the 

 Zwartkop's Eiver below Uitenhage. 



Remarks. This characteristic Trigonia, which belongs to the 

 section Scabras, may be said to exhibit in very exaggerated manner 

 some of the most striking features which render T. ventricosa 

 (Krauss) so conspicuous amongst the members of this section. 

 T. ventricosa, though so well distinguished by its form and ornamen- 

 tation, is in some respects comparable with the European T. 

 scabricola Lycett or T. aliformis Park., and was even considered 

 identical with the latter by L. von Buch.i T. kraussi is so far 

 removed from European members of the section that a detailed 

 comparison with these is unnecessary, and it only remains to draw 

 attention to the characters which differentiate it from T. ventricosa 

 and the apparently closely allied T. subventricosa which occurs in 

 the Belgrano beds of Patagonia. 



T. kraussi is principally distinguished by its large size and the 

 very prominent character and wide spacing of the anterior ribs. 

 Another striking feature is the great anterior inflation and extensive 

 flattened frontal face. In other respects the resemblance to T. 

 ventricosa is so great that I was led to ask Mr. Rogers whether he 

 was aware of the existence of specimens intermediate in character 

 between the extremes ; his reply, however, was in the negative. So 

 many specimens of T. ventricosa have been obtained by various 

 collectors, and the Uitenhage TrigonicB are so well represented in 

 European museums that we might certainly expect to find inter- 

 mediate forms, if such existed, in some of the collections. No 

 specimens connecting the extreme forms appear to have been found, 

 and amongst the Trigoncs of the Oomia beds in Cutch the representa- 

 tives of T. ventricosa, met with in abundance, agree with the smaller 

 typical form in South Africa. 



T. kraussi has approximately double the dimensions of T. ventricosa, 

 but is, perhaps, slightly shorter relatively to height. The number 

 of anterior ribs formed in the youthful and early adult stages may 

 perhaps be not widely different, but the spacing of the ribs rapidly 

 widens in the larger form, and the interspaces may reach 8 mm. in 



* Eogers and Schwarz (1), p. 10. f von Buch (1), p. 23. 



