The Invertebrate Fauna of the Uitenhage Series. 83 



to the plane of symmetry of the valves. The junction of this portion 

 of each valve with the very gently convex upper portion is acutely 

 angular, giving rise to a sharp carinal ridge which, however, becomes 

 very slightly blunted towards the posterior end of the valve. 



The shell is devoid of sculpture, but its smooth upper surface is 

 marked by numerous delicate growth-lines which form a sweeping 

 curve between the hinge-margin and the carinal ridge. The antero- 

 ventral area is sometimes marked by coarser, rounded, obliquely 

 running ridges of accretion. Traces of the narrow, elongated, 

 fossilised ligament have been observed. 



Dimensions. 



Length of the antero- ventral margin 37 mm. 



Length of the cardinal margin 16 ,, 



Greatest breadth of flank between the carinal ridge and the 



posterior valve-margin 13 ,, 



Greatest width of the antero-ventral area in one valve 7 ,, 



Occurrence. Found at Dunbrodie, Sunday's Eiver (319). Speci- 

 mens collected by Atherstone and preserved in the British Museum 

 (Natural History) are labelled "Cuylers." Also collected by Mr. 

 Eogers from the lowest beds on Zoet Geneugd, right bank of 

 Sunday's Eiver (64h), and in the Coega Valley, one mile up the 

 line from Coega station (479g). 



Remarks. This is doubtless the form noticed by Krauss :;: in the 

 lowest fossil-bearing beds on the left bank of the Zwartkop's Eiver 

 below Uitenhage, in association with Trigonia herzogi, T. conocardii- 

 formis, and T. ventricosa. Krauss speaks of a Mytilus " with a sharp 

 ridge running from the umbo to the hinder extremity and a flat 

 truncated surface extending from this ridge to the ventral margin." 

 The shell was capable of attaining dimensions exceeding those 

 of the larger specimen figured here. A specimen in the Geological 

 Society's collection, incomplete at the posterior end, must have 

 originally measured 50 mm. in length ; the maximum breadth of 

 its flat area is 11 mm. in each valve. Two imperfect large valves 

 now preserved in the British Museum (Nat. Hist.), which form part 

 of a collection at one time placed in the Museum of Practical 

 Geology, probably had a length of fully 45 mm. when complete. 

 The antero-ventral area measures about 9 mm. in breadth in one 

 of these, while the greatest breadth of the flank, measured by a line 

 perpendicular to the carinal margin, is 18 mm. The area is perfectly 

 flat and the carinal ridge acutely sharp. 



In the absence of knowledge regarding the interior of the shell, 



* Krauss (2), p. 443. 



