of the Chalk Formation. 387 
taceous. They then exhibit great diversity of form, 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 certaim forms of Ezogyre ; and hence they are of the greatest 
importance for deteymining to which of these divisions of the 
chalk a particular bed belongs. The beak of all the Ezogyre 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 Hxogyre, which 
is wanting in the Gryphee and still more in the oysters. This 
also explains why the right side of the Ezogyre is always the 
smaller one, and much less expanded than the left side. 
All the Ezogyre may be divided into two sections accord- 
ing to the form of this side with the muscle :—into those m 
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 Exogyre 
carinate, and (2) the Ezogyre expanse. Singularly enough these 
two divisions correspond to the geological occurrence of the 
genus ;—the first section, or the Hxogyre carinate, appearing 
especially in the lower; the other section, or the Exogyre ex- 
panse, mostly in the upper strata of the chalk deposits. These 
two divisions are represented in a remarkable manner by the 
Ezxogyra Couloni, or aquila, Goldfuss, and the Ex. columba ; the 
first characteristic of the Néocomien, 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 (Rémer). 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. * 
258 
