1896.] 



155 



[Farr. 



Fig. 



A very remarkable character of the lumbar vertebrae is that they have 

 spines which are nearly, if not quite, as high as those of the anterior 

 dorsal region, which in the horse are so much elongated. In the latter 

 the lumbars have spines which are lower, more nearly erect, of more 

 considerable antero-posterior extent proportionately and are much less 

 compressed transversely. 



The Sackum. 



The sacrum of M. baircli, as in most of the Ungulata, consists of one 

 broad vertebra joining the ilia, followed by a series of narrower ones, 

 gradually diminishing in width anchylosed to it behind. These latter 

 diminish in width very gradually. In living Ungulates the number of 

 vertebrte entering into the formation of the sacrum varies with the age 

 of the individual and also varies in individuals of 

 the same age. 



In the specimen which belongs with the pelvis 

 described below there are six vertebrse. This is 

 the most perfect sacrum of M. hairdi yet found, 

 and the component vertebra are fortunately well 

 preserved and hardly crushed at all (see Fig. 2 

 and Plate xiii). 



The first or true sacral vertebra is greatly ex- 

 panded transversely and bears large articular sur- 

 faces for the ilia. 



Anteriorly there are large convex facets which 

 fit into the corresponding concavities in the trans- 

 verse processes of the last lumbar vertebra. The 

 first sacral has a low and comparatively wide cen- 

 trum. The spine is very high, very much com- 

 pressed laterally, as are all the spinous processes of 

 the vertebras, and is directed strongly forward, while in the modern 

 horse it is almost vertical. The five succeeding vertebras have trans- 

 verse processes which are not so widely expanded, the centra are very 

 much depressed and the neural arches are low and gradually decrease 

 in height posteriorly. This, of course, conditions the size of the neural 

 canal, which in this region is very much attenuated. The expanded trans- 

 verse processes of the contiguous vertebrae are all united, so that they 

 form a narrow elongate plate. The spine of the second sacral is gone, but 

 the others are all preserved. That of the third is almost vertical, while 

 the spines of the three posterior sacrals all slope backward at a decided 

 angle. There is thus a very abrupt transition in the direction of the incli- 

 nation of the spines from the first in which the spine projects forward 

 to three in which the process is almost vertical. The plate formed by 

 the anchylosis of the centra and transverse processes of the vertebras is 

 concave inferiorly or curves downward posterior to first sacral. The 

 sacrum presents inferiorly the foramina for the five pairs of sacral 



PROC. AMER. PHILOS. SOC. XXXV. 151. T. PRINTED SEPT. 2, 1896. 



Sacrum of M. baiedi, 

 Inferior view. 



