The Invertebrate Fauna of the Uitenhage Series. 119 



less noticeable extent in T. van,, and the bending is absent in some 

 individuals. 



When studied in connection with the shells of the group of 

 Trigonia v-scripta from the Oomia beds of Cutch, T. stoioi can only 

 be closely compared with T. v-scripta itself. Though, when adult, 

 it resembles this in most striking manner in the backward position 

 of the umbones, the posterior elongation and general form, and 

 especially in the character of the flank ornamentation, T. stoioi may 

 be readily distinguished by its relatively more elongated and poste- 

 riorly attenuated figure, and its anteriorly more acutely converging 

 upper and lower margins. T. v-scripta is a shorter shell relatively 

 to height, particularly in the late adult and fully grown state, while 

 its valves are somewhat flatter and less inflated. In the youthful 

 stage the two are widely different in character ; T. v-scripta then has 

 a very few coarse concentric flank-ribs, which, continuing across 

 the well-marked carinal angle, pass obliquely forwards over the area 

 in the form of attenuated thread-like raised lines, and terminate at a 

 delicate linear ridge which represents an inner carina. Similar 

 characters distinguish the young T. rccarva, another Oomia form. 

 This, like T. stoiui, has strong posterior elongation in the adult, and 

 a smooth tract on the posterior portion of the flank ; but it is well 

 contrasted by its much smaller size, its rounded convex frontal 

 profile, its more crowded and much less steeply sloped anterior ribs, 

 and its imperfectly developed, short posterior ribs. 



T. hcterosculpta Stanton,* from the Belgrano beds (Lower Cre- 

 taceous) of Patagonia, is very probably a related form. Its youthful 

 characters are closely similar to those of T. stoiui, but the adult shell 

 is much shorter, more triangular in form, and much less elongated 

 posteriorly, w T hile there are marked differences in the details of 

 sculpture. 



TKIGONIA CONOCARDIIFORMIS (Krauss). 

 Plate VII., figs. 2, 2, 25, 3, 4. 



1843. Lyriodon conocardiiformis F. Krauss, Amtlicher Bericht liber 



die zwanzigste Versammlung der Gesellsch. deutsch. 



Naturforscher und Aerzte zu Mainz im Sept., 1842, 



p. 130. 

 1850. Lyrodon conocardiiformis F. Krauss, Nov. Act. Acad. Cass. 



Leop. -Carol. Nat. Cur., vol. xxii., pt. 2, p. 454, Tab. 49, 



figs. la-Id. 



* Stanton (3), p. 20, pi. iv., figs. 16-18. 



