The Invertebrate Fauna of the Uitenhac/e Series. 113 



all the individuals examined considerably exceed the dimensions at 

 which this feature becomes noticeable in T. van. . . . Sharpe's type 

 is of larger dimensions than the known specimens of T. Jitbia.' 



The extent to which an unsculptured space near the lower part of 

 the frontal margin may be present in T. ran is really somewhat 

 variable, but a few of the lowest ribs of the anterior series seem 

 always to fail to extend at their upper terminations to the valve 

 margin. In some individuals, two or three of these lower ribs may 

 be slightly bent down at their anterior ends in manner reminiscent 

 of the strong, angular bending at the front end of these ribs in 

 T. stowi, as well as in the Indian T. v-scripta. It may also be 

 noted that although the inclination of the frontal series of costae in 

 T. van is always as steep as in the specimen depicted in Sharpe's 

 figure, yet these ribs may in some instances attain a stronger 

 development and may be of a rather more robust character. 



A shell possibly related to T. van is T. kiihni G. Miiller,* from 

 Neocomian strata at a locality 23 km. west-south-west of Mtshinga 

 in German East Africa. To judge from the description and figures, 

 this is distinguished by an angular rib arrangement very similar to 

 that exhibited by the members of the group of T. v-scripta in India, 

 and T. van and T. stowi in South Africa. T. kiihni seems to differ, 

 however, by its less equilateral form and the persistent ornamenta- 

 tion of its area. The figure of an imperfect specimen of T. kiihni 

 (fig. 8) shows a curved frontal profile and the ribs of the frontal 

 series obliquely crossing the growth-lines in manner that recalls 

 the same characters in T. van, though still more reminiscent 

 of T. rccurva Kitchin, from the Oornia beds. It is as yet not 

 possible to say, however, whether T. kiihni is more nearly related to 

 T. van or to the group of T. v-scripta, though it may be anticipated 

 that a study of the youthful characters will ultimately throw light on 

 this point. 



Trigonia hetcroscidpta Stanton,t from the Belgrano beds (Lower 

 Cretaceous) of Patagonia, is very probably closely related to T. van, 

 but differs by the much shorter and more elevated triangular figure. 

 The first-formed ribs of the posterior series are vertical or backwardly 

 inclined, and not forwardly sloped as in T. van, while several 

 successive anterior ribs terminate abruptly on the flank of a single 

 vertical posterior rib. In the neanic stage, however, the plan of 

 sculpture as well as the form of the valve appears to agree closely 

 with that of T. ran. 



* G. Miiller (1), p. 501, Taf. xxv., figs. 0-8. 

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



