’ 
O. C. Marsh —Vertebre of Recent Birds. 269 
since these vertebrae are really dorsals, and have evidently 
gradually coalesced with the true sacral vertebra. 
In the caudal vertebrae of recent birds we have, in a 
essentially the same, and in the fossil species the articulations 
at least appear to follow the general rule. In Pavoand Geoccocyx, 
the caudal vertebre exhibit a tendency to a proccelian union. 
Some other forms also show unimportant modifications of 
the normal type of caudal articulation, but nothing to suggest 
a real objection to the explanation now proposed of the origin 
of the vertebree characteristic of Birds. 
In bringing together the above facts, and others suggested 
by them, the classification and development of the various 
forms of vertebrze appear to be somewhat as follows: 
(1.) Biconcave vertebre (Fishes and ba the primi- 
tive type; a weak articulation, admitting free, but limited 
motion. From this form, have been directly derived the other 
varieties, namel 
Plane pened mee affording a stronger joint, 
with motion still restricted. 
8.) Cup-and-ball eee (Reptiles); a strong and flexible 
joint, well fitted for general motion, and evidently produced 
by it. e vertebrae are proccelian when lateral motion 
is dominant (Serpents); opisthocoelian with varied motion 
(Dinosaur cervicals 
(4.) Saddle iehicbte (Birds) ; the highest type; a very strong 
and free articulation, especially adapted to motion in a vertical 
plane, and mainly due or riginally to its predominance. 
This subject will be more fully discussed and illustrated by 
the writer in a future communication 
Yale College, New Haven, Conn., Feb. 25th, 1879. 
