OSTREA. 349 



Valves with strong, usually angular folds which, in the elongate forms, extend 

 from the median line to the margin ; the folds are usually somewhat curved, but 

 may be almost straight; the amount of their curvature decreases in passing from 

 the earlier to the later part of the shell. The margins of the two valves interlock 

 by means of sharply pointed tooth-like projections formed by the interspaces 

 between the folds. On the median ridge of the valves the folds are usually 

 irregular, and may bear on each side of the ridge short spines ; near the margins 

 of the valves long tubular outgrowths occur in some specimens. The number and 

 size of the folds vary considerably in different individuals; often the later folds 

 are stronger than the earlier ones, and those on the convex side stronger than 

 those on the concave side. The part of the right valve near the umbo is smooth 

 and without folds so long as the corresponding part of the left valve is attached ; 

 this portion is commonly small, but becomes extensive when the attached surface 

 of the left valve is large, and then the folds are limited to the relatively small 

 marginal part of the valves. The triangular ligament-pit may be slightly or con- 

 siderably curved. The adductor impression is oval, and usually not far from the 

 hinge-line. 



Affinities. — The examples of this species found in the Lower Cretaceous have 

 been named 0. rectangularis, Romer, and 0. macroptera, Romer. Pictet and 

 Campiche, de Loriol, and Weerth regard 0. macroptera as distinct from 0. rect- 

 angularis, and state that the former differs from the latter principally in the 

 relatively smaller height of the shell, the larger posterior wing, and the larger 

 ribs. D'Orbigny, Maas and Wollemarm, on the other hand, consider that the two 

 forms cannot be separated. The last author, after studying a large collection of 

 specimens, states that the height of the shell depends mainly on the age of the 

 individual ; that the size of the posterior wing varies greatly, being in some cases 

 quite small, in others very large, and between the two extremes every gradation 

 may be found. The size of the ribs likewise varies. The study of numerous 

 English specimens leads me to endorse Wollemann's view. Pictet and Campiche 

 figured as 0. macroptera a small example from the Lower Greensand of Atherfield, 

 but larger specimens from the same horizon possess a higher shell like 0. rect- 

 angularis, and cannot be separated from 0. macroptera. 



The examples found in the Lower Cretaceous deposits have been generally 

 regarded as distinct from those in the Upper Cretaceous, which in this country 

 have been usually named O.frons or 0. carinata. The principal distinction is said 

 to be the larger posterior wing in the Lower Cretaceous form. But the study of 

 a large series of specimens shows that in both the Upper and the Lower Cretaceous 

 the size of the wing varies greatly, as well as the height and curvature of the 

 shell, and the number and coarseness of the ribs. As a rule, however, the posterior 

 wing is better developed in the Lower than in the Upper Cretaceous examples, 



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