Farr.] J OO [May 15, 



nerves, the inferior sacral foramina, while above we also find laterally 

 between the nem-al arches of the contiguous vertebrte the five pairs of 

 the superior sacral foramina. 



Measurements of the Sacrum. mm. 



Length 116 



Extreme width 64 



Width third sacral 23 



Width fourth do 21.5 



Width fifth do 20 



The Caudals. 



The few caudal vertebrae preserved are sufficient to give us a general 

 idea of the character of the tail. The first caudal has very widely ex- 

 panded transverse processes similar to those of the posterior sacral re- 

 gion ; the centrum is oval and the neural arches arise at a very great 

 angle enclosing a high and very narrow neural canal. The transverse 

 processes are of considerable antero-posterior extent, but do not equal 

 the length of the centrum in width as they do in the posterior A^ertebrse 

 of the sacral region. It is not possible to determine how many of the 

 caudal vertebrae had complete arches, because of incomplete material. 

 In Equus* the spine of the neural arch is bifid in the second caudal and 

 the arches are incomplete on the third. The transverse processes gradu- 

 ally become shorter, the neural arches more rudimentary and are finally 

 lost, and all we have is a cylinder of bone with very rudimentary pro- 

 cesses which gradually diminish in size. Among the caudals preserved 

 is one of these last, in which all the processes are very feebly developed. 

 All the vertebrae of the tail are in general like those of the horse, and in 

 them, as in most all of the anatomical features, we see a foreshadowing 

 of what the future horse is going to be. 



The Sternum. 



With tlie almost complete skeleton figured in the restoration of M. 

 bairdi in Plate xiii are preserved tliree segments of the sternum. These 

 are the xiphisternum and two segments of the mesosternum. The 

 former is very much more elongate and not so high as the other divisions 

 of the sternum. Anteriorly it is about twice as broad as high, while 

 posteriorly it is A'ery much flattened. The free border is thin and 

 rounded with irregular surface, showing where cartilage was attached. 

 Laterally the body of this segment as of all the other is concave. The 

 superior border is almost plane, while the inferior is slightly concave, or 

 the free end may be said to project slightly downward. 



The next segment in front of the above that is preserved is very evi- 

 dently the penultimate segment of the mesosternum. This is very differ- 

 ent in shape from the xiphisternum. The posterior portion is wide and 



* No, 33P, Princeton Coll. 



