OSTREA. 375 



Description. — Left valve usually thick, very convex, gryplieate, more or less 

 oval, height usually considerably greater than the length ; slightly or moderately 

 inequilateral, sometimes with a rounded ridge extending from the umbo to the 

 postero-ventral extremity. Posterior part often convex and separated from the 

 remainder of the valve by a furrow. Umbo sharp, prominent, more or less 

 incurved, and usually with a small posterior curvature ; surface of attachment 

 generally small. Area and ligament-pit high, straight or with a small backward 

 curvature. Adductor impression oval or rounded. Right valve thin, concave. 

 Surface of both valves smooth, except for growth-lines. 



Affinities. — This species is closely allied to 0. vesicularis, but the umbo is more 

 pointed, usually less incurved and with a smaller attached surface ; the area is 

 higher, and usually the height of the shell is relatively greater in proportion to its 

 length. Most of these differences are probably to be accounted for by the small 

 size of the attached surface. 



Type. — From the Upper Greensand of Warminster, in the British Museum. 



Distribution. — Upper Greensand : common in the zone of Scldcenbachia rostrata 

 of Ventnor, Warminster, Dinton and Potterne; rare in the zone of Pecten asper 

 of Ventnor, Warminster, and Ballard Down (Swan age). Chloritic Marl of 

 Compton Bay, Isle of Wight. 



Ostrea canaliculata (Sowerby), 1813. Plate LVI, figs. 2-16. 



1813. Chama canaliculata, J. Sowerby. Mm. Conch., vol. i, p. 68, pi. xxvi, fig. 1 



(non Ostrea canaliculata, Sowerby.) 

 1821. Ostrea canaliculata, M. J. L. Defrance. Diet. Sei. nat., vol. xxii, p. 26. 

 1827. Ostrea lateralis, S. Nilsson. Petrifie. Suecana, p. 29, pi. vii, figs. 7 — 10. 



— Chama conica, Nilsson. Ibid., p. 28, pi. viii, fig. 4. 



1829. GtRyph^a canaliculata, /. de C. Sowerby. Min. Conch., vol. vi, p. 218. 



— Exogyra undata, Sowerby. Ibid., vol. vi, p. 220, pi. dev, figs. 5 — 7. 

 1833. Ostrea lateralis, A. Gohlfuss. Petref. G-erm., vol. ii, p. 24, pi. lxxxii, 



fig. 1. 

 ? 1834. Gryph.ea vomer, S. G. Morton. Synopsis Organic Remains Cret. U. States, 



p. 54, pi. ix, fig. 5. 

 1837. Ostrea lateralis, W. Hisinger. Lethsea Suecica,. p. 46, pi. xiii, fig. 1. 



— Amphidonte undata, G. G. Pusch. Polens Paliiont., p. 39. 



1842. Exogyra parvula, A. Leymerie. Mem. Soc. geol. de France, ser. 2, vol. v, 



p. 17, pi. xii, figs. 8, 9. 

 1845-6. Ostrea lateralis, H. B. Geinitz. Grundriss d. Verstein., p. 480, pi. xx, 



fig. 22. 



1846. Exogyra lateralis, A. E. Eeuss. Die Verstem. der bohm. Kreideforniat., 



pt. 2, p. 42, pi. xxvii, figs. 38—47. 



1847. Ostrea canaliculata, A. d'Orbigny. Pal. Franc. Terr. Cret., vol. iii, 



p. 709, pi. cccclxxi, figs. 4 — 8. 



