Nigerian Eocene Mollusca. 63 



adductor scar marking very large, rounded, ventral, covered with concentric 

 growth-lines and finer interlineations and more or less obsolete radial striations; 

 sculpture irregularly concentric, finely laminate, furnished with numerous 

 slightly elevated, radial and obscure rounded costae, superficial layer of the 

 adherent surface, consisting of delicate concentric laminae and close micro- 

 scopically small radial plications ; radial costae with colour markings sometimes 

 preserved. Upper valve oblong, nearly horizontal above, large rounded cavity 

 at each corner below ligament region ; ligament area broad, projecting in 

 front, marked with equi-distant concentric and longitudinal striations as in 

 larger valve ; external sculpture resembling that of other valve, i.e., bearing 

 obscure evidence of radial costae. 



DIMENSIONS. 



Lower valve (largest example, with 

 slightly imperfect ventral margin) 



Upper valve (another individual, 

 with imperfect antero-lateral 

 margin) 



Upper valve (probable) (of a much 

 larger example, but imperfect) 



Length 



Height 



Depth 



Length 



Height 



Depth 



Length 



Height 



Depth 



90 millimetres. 

 165 

 53 

 70 



I2O 



37 



'I approx- 

 J imate. 



125 



240 



40 



REMARKS. This species belongs to the ' longirostriform ' oysters or those 

 with a much produced ligament region, which are of frequent occurrence in 

 Tertiary deposits and of much assistance in determining the stratigraphical 

 value of the beds containing them. One of the oldest forms is Ostrea angusta, 

 Deshayes, peculiar to the French Eocenes (Ypresian Sables de Cuise), 

 although the more typical include : O. longirostris, Lamarck from the 

 Oligocene (Stampian), O. aginensis, Tournouer ( = O. crispata, Goldfuss pars] 

 from the Miocene (Aquitanian), besides O. gingensis, Schlotheim and O. 

 gryphoides, Schlotheim ( = O. crassissima, Lamarck) of Miocene (Vindobonian) 

 age, the latter species occurring, as well, in Recent seas *. The more elongate 

 of these species is O. gryphoides, which sometimes attains a perpendicular 

 measurement of 54 centimetres, including a ligament region of 23 centimetres, 

 or about nine inches, and which was described and figured by Hoernes 2 

 from the Vindobonian beds of the Vienna Basin. The species from Southern 



1 R. B. Newton and E. A. Smith. On the Survival of a Miocene Oyster in Recent Seas : Records 

 Geol. Surv. India, 1912, Vol. 42, pp. 15, pis. 1-8. 



2 Abhandl. K.K. Geol. Reichs. 1870, Vol. 4, p. 455, pis. 81-84. 



