of the Chalk Formation. 387 



taceous. They then exhibit great diversity of fornix but are at 

 the same time so remarkably regular in their geological position, 

 that each of the four divisions of the chalk may be distinguished 

 by certain forms of Fxogyrce, ; and hence they are of the greatest 

 importance for determining to which of these divisions of the 

 chalk a particular bed belongs. The beak of all the Exogyr(B is 

 very much twisted, sometimes turned round two or three times 

 like a ram^s horn. This twisting is always found on the right 

 side of the deeper (lower) valve, when the hinge or beak is turned 

 towards the observer. On this side of the beak also there always 

 appears a more or less distinct, narrower or broader depression, 

 which is continued from the margin to the beak. In the inte- 

 rior of the shell this depression forms a swelling on which the 

 muscular impression is seen. Hence it is evidently the muscle 

 that pulls up the mantle and along with it also the shell which 

 it produces, and thus causes the furrow or depression on the ex- 

 terior surface. This is a chief character of all the Exogyrae, which 

 is wanting in the Gryph(2a and still more in the oysters. This 

 also explains why the right side of the Exogyra is always the 

 smaller one, and much less expanded than the left side. 



All the Exogyr(s. may be divided into two sections accord- 

 ing to the form of this side with the muscle : — into those in 

 which this side is vertical to the surface of greatest expansion in 

 the shell, and then forms a sharp keel on the back, — and those 

 in which this side expands somewhat in a wing-like form, with 

 a rounded and not a projecting back ; that is (1) the ExogyrcB 

 carinatce, and (2) the Exogyra expansa. Singularly enough these 

 two divisions correspond to the geological occurrence of the 

 genus; — the first section, or the Exogyrce carinata, appearing 

 especially in the lower ; the other section, or the Exogyra ex- 

 pansce, mostly in the upper strata of the chalk deposits. These 

 two divisions are represented in a remarkable manner by the 

 Exogyra Couloni, or aquila, Goldfuss, and the Ex. columba ; the 

 first characteristic of the Neocomien, the second of the upper 

 chloritic chalk [chalk-marl] . Throughout the whole of Europe 

 this Exogyra columba is never looked for in vain, but in North 

 America it has not yet been seen except in Texas, and there only 

 of a small size (Romer). Its place appears to be supplied by 

 the large, scaly, wide-expanded Exogyra costata, Say, which, ac- 

 cording to Morton's statement, occurs wherever the cretaceous 

 strata are found. On the other hand, the absence of Exogyra 

 Couloni from this portion of America is quite in accordance 

 with its position in the lower beds which are wanting in North 

 America. 



25^ 



