4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



SO that all the processes alike are directed upward. The zygapophyses 

 become reduced and in large part lose their mutual articulations. 



The fore limb, which no longer comes in contact with the earth or 

 bears any load, is changed into a flipper whose single function is that 

 of striking against the water. The entire arm becomes an oar blade. 

 Of the fore limb's articulations the shoulder joint only is used; it 

 retains its ball-and-socket structure. All the other articulations are 

 held stiff. They degenerate, become flat and immovable, or are 

 wholly effaced. Practically the only function of the upper arm is to 

 support the forearm and hand. It becomes short and heavy. Its 

 middle portion retains its terete form, but its lower end is com- 

 pressed in agreement with the bones of the forearm. The radius 

 and ulna become very^ simply-formed, compressed bones, losing 

 muscle crests, sinew furrows and all pronounced articular sur- 

 faces; even the olecranon may wholly disappear. The mutual posi- 

 tion of the two bones is somewhat altered, so that they eventually 

 lie exactly fore and aft of each other. The hand is set somewhat 

 supine, fore edge downward. The carpal bones become compressed, 

 or more correctly flattened, pieces [like sections of a mosaic]. They 

 are rather indifferent as to form and number, and are immovable. 

 The folds of skin between the fingers are lengthened out to the finger 

 tips ; and the hand stiffens. The claws disappear. The first and 

 fifth fingers are somewhat inclined to be stunted, but the other fingers, 

 particularly the second, tend to lengthen and to form new joints at 

 their tips, so that the number of phalanges may increase far beyond 

 the typical three. The metacarpals and phalanges are shaped almost 

 alike, as more or less flattened pieces of bone.^ The shoulder blade 

 degenerates only slightly. In the most primitive whales it already 

 has the form which, with few exceptions, is found among the highest. 

 It is broadly fan-shaped, with a prominent, antrorse acromion, and a 

 large coracoid, but on the other hand almost without crista scapulce. 

 Rarely it becomes narrower or lacks both acromion and coracoid. 



The fact that the fore limb does not act as a support for the body 

 results in lessening the limb's pressure on the chest. Another result 

 is that the spinous processes on the anterior thoracic vertebrae lose 

 their special height. Still another result is that the connections 

 between the ribs and both the thoracic vertebrae and the sternum have 

 a tendency to become loose or to disappear. Perhaps this tendency 

 is also brought out by the fact that the water pressure on the chest 

 during diving changes strongly. The ribs may lose the capitulum, 

 and the costal cartilage may practically disappear. When this happens 



