No. 2.] THE HARD PARTS OF THE MAMMALIA. 207 
Mammalia in the necks of certain genera of monkeys. The 
majority of Mammalia present the plane articulation of all the 
vertebral centra. 
In Mammalia in which movement of the vertebre on each 
other has become impossible, the centra codssify. Such is 
their condition in the extinct Edentata of the family Glyptodon- 
tide, where the carapace is, as in the tortoises, inflexible, and 
which therefore limits the possibility of motion of the vertebral 
column. Another illustration is seen in the necks of the Balz- 
nid Cetacea, and to some degree in the Delphinidz and Physe- 
teridz. The lack of present mobility of this part of the column 
is due to its extreme abbreviation, a character which has been 
gradually developing during Czenozoic time; since the earliest 
Cetacea had considerably longer necks than the later ones, and 
had their vertebral centra distinct. I cannot account posi- 
tively for this shortening of the neck, but will make a sugges- 
tion which may prove to be sufficient. It appears to me 
probable that the shortening was the result of disuse. This 
disuse would arise from gradually increasing powers of locomo- 
tion through the water, a progress which, judging from the 
character of the limbs of the Zeuglodon, was evidently made 
after the time of the Eocene. The increase of speed would 
enable the animal to overtake and capture its prey, without the 
necessity of using a long prehensile neck in seizing it in the 
pursuit. 
The ball-and-socket articulation of the vertebrz is well known 
to be the predominant condition in the Reptilia, and the fact 
that it is necessarily associated with the flexibility of the column 
is equally well understood. This flexibility is directly as the 
weakness of the limbs, for in the large terrestrial Reptilia of 
the order Dinosauria, the vertebral articulations of the dorsal 
region, at least, are plane. That it is chiefly confined to, and 
best developed in, the most flexible regions of the column of the 
Mammalia also shows this necessary connection. There can 
be no doubt but that the ball-and-socket vertebral articulation 
has been produced by constant flexures of the column in all 
directions, as has been suggested by Marsh. 
The saddle-shaped articulation permits great lateral and verti- 
cal flexure, but none in any other direction. Longitudinal 
torsion is impossible. It is a more secure articulation than the 
