514 GEOLOGICAL HISTORY OF THE GULF OF MEXICO. 
tension. In flexion, the radius is pushed forward, and pro- 
jects somewhat beyond the end of the ulna, impinging upon the 
radio-carpal bone (scapholunar), and pushing the pinion around 
the centre of motion of the wrist-joint so that it is more or less 
flexed. In extension, the reverse motion takes place, from the 
pulling back of the radius. The proposition is carefully demon- 
strated, illustrated with three figures, and likewise shown to be 
susceptible of ocular proof by direct experiment. Several inter- 
esting corollaries are also drawn. Some such mechanism is shown 
to be an anatomical necessity, from the structure of the wrist- 
joint, to provide for the extremes of adduction and abduction that 
take place in the wrist, without straining the joint. Another 
obvious purpose subserved is equalization of muscular power, by 
relegating a part of the work, that the hand muscles would other- 
wise have to perform, to the larger flexors and extensor of the 
upper arm; and an actual saving of a certain amount of muscular 
effort, this being replaced by automatic movements of the bones 
themselves. Having seen no account of this mechanism, the 
author is inclined to think it may be unnoticed.* It is at any rate 
a new explanation of the design of the peculiar shape and position 
of the radial articulating surface of a bird’s humerus, far more 
important than that hitherto assigned, viz.: its causing simply the 
well-known obliquity of flexion of the forearm. 
ON THE GEOLOGICAL History or THE Gute or Mexico.—Byr 
Pror. Eve. W. HILGARD. 
Tuis paper, accompanied by a geological map, treats of the 
formations that have gradually filled up the ancient Mississippi 
embayment, existing after the upheaval of the Palæozoic rocks; 
whose vertex, a few miles above the junction of the Ohio and Mis- 
sissippi, is marked by the small Tertiary area in Illinois. The au- 
thor hopes that a close comparison of these deposits with those of 
the Far West, with which they were and partly still are connected, 
may lead to the more accurate parallelization of the latter with the 
marine deposits of more distant regions. Most of the subject 
Tt 
* It is indeed not mentioned in th ks of Cuvier, Meckel, Tiedemann, Wagner, and 
other distinguished authors; but Dr. Bergmann, of Gottingen (Archiv. fur Anat- 188% 
296), speaks of essentially the same thing, although the results of the mechanism are 
not so fully shown, 
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