EXOGYRA. 



Ki:! 



Exogyra imbricata, Krauss, 1 from the Uitenhage Series, is closely allied to E. 

 Couloni. 



Uemarlcs. — The proportion of the height to the length of the shell varies con- 

 siderably. In some specimens the height is much greater than the length, but in 

 others the two diameters are nearly equal. In the latter case the left valve is usually 

 less convex relatively than in the higher and more strongly carinate forms. In the 

 specimens in which the carina reaches the margin the postero-ventral extremity is 

 more angular than in those in which it becomes indistinct. The slope of the pos- 

 terior part of the left valve is gentle in the forms in which the height and length arc 

 nearly equal, but becomes steeper in the higher and more strongly carinate forms, 



Fig. 214. — Exogyra sinuata (Sow). Speeton Clay (probably zone of Belcmnites lateralis), Sj)eeton. 

 Sedgwick Museum. Left and right valves. x j. 



and is sometimes, as in the example figured by Phillips (1822), perpendicular to 

 the plane of the valves (fig. 211). In that type, which is an extreme example, the 

 umbo is only slightly curved, but this form passes gradually into others with a 

 strongly curved umbo. Indications of radial folds are seen in some specimens, 

 but they are less distinct than in some foreign examples. 



Some specimens show that the stage in which the posterior margin is sinuous 

 is preceded by one in which it is only slightly concave (fig. 199). The type of 

 Gri/phdea aquila, Brongniart, is a small example of the sinuata form with the carina 

 continued to the margin. The specimen figured by Pictet and Roux is similar, but 

 larger, and with the carina becoming indistinct towards the margin ; the large 



1 'Nova Acta Acad. Cses. Leop. -Carol. Nat, Cur.,' vol. xxii (1850), 2, p. 460, pi. 1, fig. 2. Sharpe. 

 'Trans. G-eol. Soc.,' ser. 2, vol. vii (1856), p. 197, pi. xxiii, fig. 3. Kitcliin, 'Ann. S. African Mus.,' 

 vol. vii (1908), p. 77. 



