The Invertebrate Fauna of the Uitenhage Series. 117 



though imperfectly demarcated from the area, it is marked by its 

 excavated form and smooth surface. Ridges of growth which cross 

 the area are replaced by very delicate and scarcely perceptible lines 

 of accretion on the escutcheon. At a distance of 15 mm. from the 

 um bones the escutcheon in each valve is fully as broad as the area. 

 The ligament pit is relatively short and broad. 



Dimensions. (1) (2) (3) 



Greatest length 80 . 95 . 110mm. 



Height, measured from the umbo 42 . 44 50 



Greatest depth of a single valve ... 14 ..15 . 20 ,, 



Occurrence. Marine Beds of Sunday's and Zwartkop's Eivers. 

 Specimens in the collection of the Geological Society of London are 

 labelled " Zwartkop River" (Rubidge) ; " McLoughlin's, bed No. 5 " 

 (Stow) ; " Above Modder Drift, No. 3 bed " (Stow).- In the British 

 Museum (Natural History) a fine specimen is labelled " Sundays 

 River, Pont." A specimen sent to me from the South African 

 Museum is believed to come from the Sunday's River cliffs. An 

 immature specimen, 15 mm. in length, and a portion of the flank of 

 an adult in the form of an external mould, resembling T. stowi, were 

 collected by Messrs. Rogers and Schwarz at Walton's Farm on the 

 Sunday's River, just below Dunbrodie (307). These may probably 

 represent T. stowi, though they might equally well be ascribed to 

 T. vau so far as their characters allow of determination. Another 

 immature specimen, from the railway cutting between milestones 

 24^ 24f on the Uitenhage-Graaff-Reinet railway (309) may also 

 perhaps belong to this form. Mr. Rogers collected T. stowi in a 

 small kloof, three miles up the left bank of Sunday's River (17h). 

 A hand-specimen containing one complete and two fragmentary 

 valves was also obtained by him from a cliff W. 20 S. from Comley's 

 house, right bank of Sunday's River (90h). 



Remarks. This form shows some variation in regard to the shape 

 of the frontal profile and the position of the umbones in relation to 

 the anterior extremity. In some examples, the anterior sculpture is 

 more robust and more regular than in others. The thickened and 

 downwardly directed terminations of the anterior ribs are some- 

 times a very prominent feature, in other cases they are less 

 noticeable. In some individuals the anterior ribs are very 

 closely spaced, and towards the lower part of the adult valve may 

 be much broken up into nodes (as exemplified by specimen 17h). 

 Sometimes a small portion of the flank, most anteriorly situated, is 



* See Stow's table of cliff-exposures at localities on " Upper Sundays River " and 

 " Lower Sundays River " ; Stow (1), tig. 3. 



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