144 Annals of the South African Museum. 



figured here, is so well preserved that these surface characters are 

 plainly visible to the naked eye. 



The occurrence of a representative of this exclusively Cretaceous 

 genus has considerable interest when regarded as additional evidence 

 for the age of this molluscan fauna. It is perhaps fortunate that the 

 shell before us, when well preserved, is sufficiently characterised by 

 its own distinctive form to preclude confusion with European mem- 

 bers of the genus, especially as the nomenclature of these has for so 

 long been in an unsatisfactory state. The European forms appear 

 to show considerable variation, and are often preserved merely as 

 casts, while the delicacy of the shell renders it liable to distortion, 

 and true characters of similarity or difference are, therefore, often 

 difficult to establish. In like manner, immature or ill-preserved 

 specimens of Th. papyracea may seem to differ somewhat widely 

 from the large individual figured by Sharpe, and may appear to 

 approach some of the European forms ; hence, a brief comparison 

 with certain of these may perhaps be not without value. 



The shells from the Blackdown Beds described under the name 

 Corbula Icevigata by Sowerby* seem to represent the same form as 

 that afterwards named Thetis major,} while some shells from the 

 Lower Greensand of Atherfield (Isle of Wight) cannot always be 

 satisfactorily distinguished from small specimens of the Blackdown 

 form. Others from the Lower Greensand in the Isle of Wight, 

 named Thetis minor by the same author, J are frequently preserved 

 as casts (the specimens from Shanklin), but appear to be usually 

 well distinguished from those above-mentioned by their greater con- 

 vexity, their more prominent umbones, and frequently straighter 

 and longer hinge-line. Some individuals, however, are very difficult 

 to separate, and Eoemer united these two forms under the name 

 Thetis soiverbyi. Forbes, again, divided Eoemer's Th. sowerbyi into 

 varieties minor and major ; || but these are both regarded by Mr. 

 Woods as synonymous with J. de C. Sowerby's Thetis minor. 

 Mr. Woods also considers that d'Orbigny's Thetis lavigata is 

 identical with Sowerby's Th. minor. Despite some difficulty in 

 the comparison of certain individuals, we may on the whole satis- 

 factorily distinguish between Thetironia Icevigata (J. Sow.) which 

 includes Th. major (J. de C. Sow.) and Th. minor (J. de C. Sow.), 

 which includes Th. Icevigata (d'Orb.) 



* J. Sowerby (1), vol. iii., Tab. 209, figs. 1, 2 (1818). 



t J. de C. Sowerby (1), vol. vi., Tab. 513, figs. 1-4 (1826). 



I Ibid., vol. vi., Tab. 513, figs. 5, 6 (1826). 



F. A. Roemer (2), p. 72 (1841). || Forbes (1), 242. 



