142 Annals of the South African Museum. 



Tancredia schwarzi, from which it differs by its greater elongation 

 and relatively reduced height ; it has a more obtuse umbonal angle, 

 and the umbonal region is consequently considerably less prominent. 

 Although there is also some resemblance to shells which have been 

 ascribed to the genus Palceomya Zittel and Goubert,* Pal&omya 

 deshayesi Zittel and Goub., from the Corallian, upon which the 

 genus was established, is a more elongated shell and is more 

 inequilateral and less definitely carinated posteriorly. Its umbonal 

 angle is also very much more obtuse. Palaomya autissiodorensis 

 (Cott.) de Loriol,f from the Portlandian of the Yonne, while more 

 equilateral and more sharply carinated than the last, is also 

 relatively much more elongated than the African shell, and has 

 much less sloping upper outlines, with inconspicuous umbonal 

 region and obtuse umbonal angle. It may be noted that Miss E. G. 

 Skeat I has referred this Portlandian shell to the genus Tancredia, 

 and expressed the belief that these supposed separate genera may 

 have to be united ; she points out that they appear to agree very 

 closely both in external and internal characters. 



GENUS THETIEONIA R Stoliczka. 

 THETIRONIA PAPYEACEA (Sharpe). 



Plate VII., figs. 11, lla. 

 1856. Ceromya papyracea D. Sharpe, Trans. Geol. Soc. Lond., ser. 2, 



vol. vii., p. 195, pi. xxii., fig. 8. 



Supplementary Description. The shell is of rounded sub-quadrate 

 or oblong outline, with the length slightly greater than the height ; 

 it is strongly inflated in the umbonal half. The umbones are 

 strongly incurved and are anteriorly directed ; they are situated at a 

 distance of less than one-third of the shell's total length from the 

 anterior margin. The hinge-line is only very slightly curved and has 

 a scarcely perceptible downward slope when traced backwards from 

 the umbo, passing posteriorly by a somewhat abrupt curve into the 

 elongated posterior margin; this margin, also, is only very slightly 

 convex in profile and gives the shell an aspect of vertical posterior 

 truncation. The lower border forms a gently convex profile. There 

 is no keel on the posterior side of the valve. 



The surface of the very thin shell is ornamented by numerous, 



* Zittel and Goubert (1), pi. viii., figs. 6-8. 



t de Loriol and Cotteau (1), p. 510, pi. v., figs. 12-14. 



j Skeat and Madsen (1), p. 129. 



