72 Annals of the South African Museum. 



Dimensions. 



Greatest measurement radially from the umbo ... 64 mm. 

 Greatest length measurement, at right angles to 



the last 40 



Greatest depth of a single valve 10,, 



Occurrence. Eailway cutting between milestones 24| 24f on the 

 line from Uitenhage to Graaff-Eeinet, about three miles from 

 Uitenhage (346) ; also stated to occur at Grass Eidge. Specimens 

 in the collection of the Geological Society are labelled " Sunday 

 Eiver " (Eubidge) and " Zwartkop Eiver" (H. Longlands). Tate's 

 record of locality is "in a yellow shell-rock from the Zwartkop 

 Eiver sandstone, with Placunopsis undulata and fossil wood 

 (Eubidge)." Fine specimens sent to me from the South African 

 Museum are from the Sunday's Eiver. Mr. Eogers obtained this 

 form on the left side of the Coega Valley, half a mile down from the 

 railway (455g). 



Remarks. Tate unhappily brought this shell into comparison with 

 two British Jurassic forms, L. rigidula (Phill.) from the Cornbrash, 

 and-Zy. ovalis (J. Sow.) from the Great Oolite. Lima rigidula-'' is of 

 quite another type ; it has a widely different outline, is comparatively 

 coarsely ornamented, and has well-developed anterior and posterior 

 ears. L. ovalis, t more oblique and anteriorly produced, with 

 minutely delicate and crowded linear ornaments, can scarcely be 

 brought into close comparison. 



Lima obliquissima in reality may be most closely compared with 

 those shells to which has been applied the sub-generic name Acesta,l 

 represented in Cretaceous and later strata, and existing at the 

 present day. The characters whereby this group of forms is distin- 

 guished from Plagiostoma and other divisions of the genus have 

 been clearly set forth by E. Philippi in his analysis of Lima. 

 Although I have as yet been unable to ascertain the position and 

 form of the ligament pit in L. obliquissima, the close agreement in 

 other features can leave little or no doubt that this shell must be 

 united with Acesta, if this sub-generic group be adopted at all. 

 These features are principally seen in the great relative height of the 

 valve ; the anteriorly little-produced outline ; the anterior and ter- 

 minal position of the umbones ; the great reduction of the anterior 

 ear and the imperfectly demarcated posterior ear. The fine linear 



* Phillips (1), p. 116, pi. vii., fig. 13.' 



t J. Sowerby (1), tab. 114, fig. 3 (1815) ; Morris and Lycett (1), Part 2, p. 29, 

 pi. iii. , fig. 5. 



J H. and A. Adams (1), p. 558. E. Philippi (1), p. 630. 



