EXOGYRA. 401 



behind the umbo and is usually of small or moderate size. A carina extends in a 

 curve from the umbo towards the postero-ventral extremity, and sometimes bears 

 coarse tubercles or spiny projections ; it is at first strong 1 , and may be continued to 

 the postero-ventral margin of the valve, which is then angular ; but in large 

 specimens it often becomes indistinct during the later stages of growth and may 

 ultimately disappear. Usually the carina is angular at first and becomes rounded 

 later, but it may be angular throughout or rounded throughout; sometimes it 

 divides the valve into two nearly equal parts, but usually the anterior is larger 

 than the posterior part. The former is convex ; the latter is flattened or concave 

 or undulating, and its slope to the posterior margin is often gradual, but may be 

 steep or even perpendicular. The surface of the valve bears distinct growth- 

 lines, and sometimes shows faint radial folds. The curvature of the ligament-pit 

 varies with that of the umbo. 



Right valve usually nearly flat or slightly concave or undulating, but sometimes 

 very concave. Its outline varies as in the left valve. Umbo small, more or less 

 considerably spiral. Surface with growth-lines and occasionally with faint radial 

 ribs. Adductor impression large, oval, usually sub-median. 



Affinities. — By some authors, especially Leymerie and Coquand, the forms 

 similar to the types of Gryphaza siuuata, Sowerby, and G. aquila, Brongniart, 1 

 have been regarded as specifically distinct from those like Exogyra subsinuata, 

 Leymerie-; but Pictet and Renevier (1858), after studying a large series of 

 specimens from Switzerland, France, and England, came to the conclusion that it 

 was impossible to separate these as two species ; the same view was maintained by 

 Pictet and Campiche in 1871, and has been supported more recently by Wollemann 

 (1900), who has examined a large series of specimens from North Germany. 

 Pictet, Renevier and Campiche showed that the two forms are not, as Leymerie 

 maintained, characteristic of different horizons, but occur together, although varying 

 in abundance at different levels. The study of numerous English and some foreign 

 specimens leads me to endorse the view first expressed by Pictet and Renevier. 

 The variation is found to be extremely great, and the different forms are connected 

 by numerous gradations. It is also noticeable, as Pictet and Renevier pointed out, 

 that the varieties of subsinuata, some of which were named aquilina, dorsata, 

 and falcifera by Leymerie, may differ from one another more than they do from 

 sinuata. The forms of the sinuata (aquila) type are common in the Atherfield 

 and Hythe Beds, and those of the subsinuata type occur mainly in the zones of 

 Bclemnites lateralis and B. jaculuiu, but neither is confined to those horizons. 



1 See figs. 194, 195, 199, and the figures of Sowerby (1822), Brongniart (1822), Pictet and Roux 

 (1853), Leymerie (1846), pi. vi, fig. 1. 



2 See figs. 202, 203, 206, 212, 214, and Leymerie (1842), pi. xii, figs. 3, 7; (1846), pi. vii, 

 figs. 3, 4. 



