The Invertebrate Fauna of the Uitenhage Series. 123 



or the CostataB ; but in view of the remarkable manner in which 

 widely divergent types of Trigonia in the Oomia beds of ditch 

 show, by their neanic characters, their relationship to the Costatae, 

 it would not have been surprising to obtain evidence of a similar 

 descent in T. conocardiiformis. The young stages, however, exhibit 

 no trace of longitudinal ornamentation on the area, and it is difficult 

 to surmise its true derivation. 



T. conocardiiformis, by reason of its peculiar characters of outline 

 and sculpture, has for long held a somewhat isolated position among 

 the representatives of the genus. Lycett at first expressed the 

 belief that it is an abnormal example of the Clavellatae, only 

 remotely related to other members of the section ; * but he after- 

 wards modified his opinion and associated this form with the 

 " crenulated examples of the Scabrte." t Definite indications of 

 such a relationship, as we have seen, are not to be recognised. 



It is a point of great interest that on the South American 

 continent, an apparently very close ally of T. conocardiiformis has 

 been found to occur, and in association with two members of the 

 Pseudo-quadratae. The shells described by Burckhardt \ under 

 the name " Trigonia aff. conocardiiformis " were collected from 

 strata ascribed to the Lower Neocomian at Las Lajas (Argentine), 

 and they certainly bear a very remarkable resemblance to this 

 South African form. In general figure and outline the similarity 

 is very striking, while differences in the hinge apparatus of the left 

 valve are of a minor character. The principal points of distinction 

 are in the nature of the sculpture of the flank in the adult stage ; 

 regarding the sculptural plan in the youthful stage of the South 

 American shell, no information is as yet forthcoming. The tendency 

 in some few individuals of T. conocardiiformis to show a develop- 

 ment of two distinct series of ribs on the flank during the early 

 adult stage is illustrated in more complete and emphasised manner 

 during the whole adult period in the South American shell. Most 

 of its anterior ribs, though in continuity with ribs of the posterior 

 or vertical series, form an obtuse angle with these, and pass across 

 the flank towards the frontal border in a direction more nearly 

 horizontal than the downwardly directed anterior portions of the 

 ribs in T. conocardiiformis. The contrasted aspect of the sculpture 

 in the two forms is well illustrated in plate xiii. of Dr. Burckhardt's 

 monograph, where figures of African and South American individuals 

 are presented side by side. 



* Lycett (3), p. 121 (1875). f Ibid., p. 210 (1879). 



I Burckhardt (2), p. 72, pi. xiii., figs. 1, 2. 



