The Invertebrate Fauna of the Uitenhage Series. 73 



sculpture of the flank, it is true, recalls that of some Plagiostoma, 

 but the shell Js in other respects strongly contrasted with that 

 group. Concerning the value of the sub-generic name Acesta, par- 

 ticularly in the application to fossil forms, opinions have widely 

 differed. It was accepted by Stoliczka* and by Zittel,f and latterly 

 by Philippi, while regarded by Fischer as only representing a 

 section of Lima.\ This last author merely quotes the living type 

 species, L. excavata. G. Boehm, however, has expressed the 

 opinion that the sub-genus Acesta cannot be upheld, pointing 

 out that the original diagnosis is inaccurate ; that the name is 

 unpractical in its application to fossil forms, in many of which 

 the direction and position of the ligament pit, upon which the 

 exact determination depends, cannot be investigated; and further, 

 that the distinctive features of the proposed sub-genus are of slight 

 importance. The first of these objections can have little weight, 

 merely depending, as pointed out by Philippi, upon an original 

 error of orientation. There is something to be said for the last 

 point, perhaps, and it is clear that amongst fossil representatives 

 of the genus there are some forms which illustrate in varying 

 degrees the inequilateral character and the development of the 

 anterior auricle. The name, however, though possibly not standing 

 for a natural group, may for the present be conveniently em- 

 ployed to indicate the apparent affinities of the forms to which it 

 is applied. 



L. obliquissima shows a striking outward similarity to the typical 

 living Lima (Acesta) excavata (Fabr.) Chemn.,|j though differing 

 from this by the relatively more elongated outline, the shorter hinge- 

 line, and the more restricted posterior cardinal expansion of the flank. 

 A Neocomian shell having great resemblance in form to L. obliquis- 

 sima is Lima undata (Desh.) Leym.,*I but it is distinguished by its 

 longer hinge-line and stronger ornaments. In the shortness of the 

 superior margin and the character of the surface ornamentation, 

 Lima orbignyana Math.** shows closer agreement, but it is dis- 

 tinguished from L. obliquissima by the more perfectly demarcated 

 posterior ear, the concave outline behind this, the concave anterior 

 area and the extension of linear ornaments over the whole flank. 

 Matheron's figure shows a distinctly developed projecting anterior 



* Stoliczka (2), pp. 413-415. f Zittel (5), p. 26 (1881). 



} Fischer (1), p. 941 (1886). G. Boehm (1), p. 625. 



|| E. Philippi (1), Taf. xxiv., fig. 5. 



IF Leymerie (2), p. 10, pi. 8, fig. 8; d'Orbigny (3), p. 528, pi. 414, figs. 9-12 

 (1847). ** Matheron (1), p. 182, pi. 29, figs. 3, 4 



