140 Annals of the South African Museum. 



truncated. The oblique posterior border has a sub-angular junction 

 with the inferior border. The greatest height occurs at the umbo. 

 The valves are weakly inflated, most strongly so posteriorly to the 

 middle, and are anteriorly compressed and flattened. 



On the posterior side of the valve, a well-marked carinal ridge 

 extends obliquely backwards from the umbo to the posterior angle of 

 the shell, cutting off a narrow, flattened area from the flank. Near 

 the umbo, the surface of the area is directed at right angles to that 

 of the flank, but the angle becomes more obtuse when traced 

 towards the posterior end of the valve. The surface of the area is 

 slightly concave in the neighbourhood of the umbo, but becomes 

 flat posteriorly. The surface of the flank close to the antero-superior 

 margin curves over with a convex surface towards the margin. 

 The surface of the valves is marked only by occasional faint growth- 

 lines. 



Immature specimens have a rather more pointed anterior outline 

 than an individual of larger dimensions. 



Dimensions. (1) (2) (3) 



Length 16 . 16 22mm. 



Height, measured from the umbo 10 . 11 15 ,, 



Greatest depth of a single valve 4 . 4 . 5 ,, 



Occurrence. In the kloof east-north-east of Bed House, on the left 

 side of the Zwartkop's Eiver (324) ; this form is also found in the 

 Marine Beds of the Sunday's Eiver, where it occurs associated in 

 the same hand-specimen with Trigonia ran, Trigonia conocardii- 

 formis, Act&onina atherstoni, and other characteristic shells. A 

 specimen of this kind is from the collection of the South African 

 Museum. Mr. Rogers collected a fine example of this form from a 

 cliff W. 20 S. from Comley's house, right bank of Sunday's Eiver 

 (95h). 



Remarks. The specimens examined are unfortunately so preserved 

 in hard matrix that it has been found impossible to investigate the 

 internal characters. The outward features of the shell, however, are 

 so well marked and so characteristic, that one can scarcely suggest 

 a doubt as to the generic position, unless, indeed, it should happen 

 that we are dealing with a striking instance of parallelism, of which 

 there is so far no evidence. It is with Jurassic forms that the most 

 striking similarities are shown. 



This shell may be most aptly compared with familiar English 

 Oolitic forms. T. extensa Lycett,* from the Inferior Oolite, is very 



* Lycett (1), pi. xi., tig. 9; Morris and Lycett (1), part iii., p. 93, pi. xiii., tig. 6 

 (1855). 



