850 SKELETON 
of the Atlas to be mentioned presently, and the position of the facets 
at once distinguishes the anterior from the posterior end of the bone. 
The transverse processes articulate with the tuberculum of the corre- 
sponding rib, while the capitulum of the latter does the like with a 
knob or facet on the side of the anterior portion of the centrum. 
When the vertebre are free the centrum of each articulates with 
that of those next to it by complicated joints, exhibiting four kinds 
of configuration, in accordance with which the vertebra are distin- 
guished as 
1. Heterocelous, or those which have saddle-shaped articular 
facets. In them the anterior surface is concave in a transverse, 
but convex in a vertical, direction, while the posterior surface 
shews the condition reversed. When looked at from the ventral 
side the joints appear to be “proccelous,” but “‘ opisthoccelous ” 
when seen from the side. This heteroccelous formation is the most 
perfect one attained by the vertebral column, and is typical of, and 
restricted to, Birds. ‘There are however a few exceptional cases 
in which the joints are not heteroccelous. 
2. Amphicelous, or those in which each end of the vertebra is 
concave. ‘This is the lowest condition, and is rapidly passed 
through by recent Birds in the embryonic stage; but Archexopteryx 
seems to have had biconcave vertebre of this kind, and the dorsal 
and cervico-dorsal vertebrae of Ichthyornis were undoubtedly thus, 
while the few well-preserved cervicals of the latter indicate transi- 
tional steps towards the heteroccelous condition. Among recent 
Birds the caudal vertebree alone are occasionally more or less amphi- 
ceelous, but this may not be a primitive feature. 
3. Procelous, or concave in front—a condition found only in the 
Atlas. 
4. Opisthocelous, or concave behind, so as to receive a corre- 
sponding knob on the anterior face of the following vertebra, 
instances of which occur in the thoracic region of the Sphentsc2, and, 
though in a much less degree, in various Steganopodes, Lari, Limicole, 
Psittaci and Steatornis. 
The Proccelous and Opisthoccelous types are not to be regarded 
as fundamentally important, as they are not primary features, but 
produced by adaptation to functional requirements. Neither of them 
necessarily indicates a Reptilian descent for Birds, nor can their 
modifications be used as valid characters in determining the affinities 
of various groups of the Class. The prevalent type among Reptiles 
is the Proccelous, while Opisthoccelous vertebra are common among 
Mammals. 
The articulations of the vertebre are further complicated by 
the presence of a ring or pad of fibrous or cartilaginous tissue 
interposed between the centrum of each vertebra and of that next 
to it. These pads vary much; when fully developed they are 
