The Invertebrate Fauna of the Uitenhage Series. 145 



Thetironia papyracea is distinguished from Th. l&vigata by its 

 greater inflation, its anteriorly situated umbones, its posterior trunca- 

 tion, and the absence of strong radial striae on the posterior side in 

 the neighbourhood of the cardinal margin. It is widely separated 

 from Th. minor by its inequilateral form and long, truncated posterior 

 margin, and apparently also by the obscure lower termination of the 

 anterior limb of the angular line impressed on the cast. . 



The Cenomanian shell ascribed by d'Orbigny to Thetis major, 

 which was renamed Thetis rotomagensis by Pictet and Campiche, is 

 much more equilateral, and appears to have the umbonal region less 

 prominent and less inflated than in Th. papyracea, and if d'Orbigny's 

 figure be correct, the angular line on the cast does not extend so far 

 up towards the umbonal apex. The anterior limb of the line is also 

 bent forwards at its lower end and passes in well-defined manner 

 across the flank. 



Thetironia renevieri (de Loriol),* from the Hauterivian of Sainte- 

 Croix and Mont Saleve, has a general outline and degree of convexity 

 somewhat approaching that of Th. papyracea, but the Swiss shell 

 has the limbs of the angular line of the cast much more widely 

 diverging, and the anterior limb forwardly bent and passing relatively 

 closely to the anterior valve-margin when traced downwards ; it 

 is also more equilateral, with much greater height in relation to 

 length. 



Thetironia genevensis (Pict. and Roux),f which occurs in the 

 " Gault " of the Perte-du-Ehone, Cosne, Sainte-Croix, and other 

 places, is distinguished by its more equilateral and more circular 

 outline, and by the form of the angular line on the cast, though 

 it approaches Th. papyracea in its inflation and in the prominence 

 of the umbonal region. Thetironia prestensis (Pictet and Cam- 

 piche) I (Aptian) lacks the elongation and posterior truncation that 

 characterises Th. papyracea, and its linear markings on the cast 

 follow a widely different course. 



Thetironia sanctce-crucis (Pict. and Camp.) is still further removed 

 from the African shell, not only by its outline, but especially by 

 the great relative breadth and shallowness of the posterior sinus on 

 the cast. 



A shell which much more closely approaches Th. papyracea in the 

 general form and outline and in the position of the umbones is 



* de Loriol (1), p. 65, pi. ix., fig. 11. 



f Pictet and Koux (1), p. 420, pi. xxx., fig. 2 (1852). 



