Farr.] 1^^ [May 15, 



its fellow of the opposite side at this point forming part of the symphysis. 

 Above the acetabulum the border of bone is high and rounded, but is not 

 sharp and angular as in the horse. The obturator foramen in the pelvis 

 of the latter is rounded and shorter in proportion to its width than in 

 M. bnirdi, being only slightly elongate, while in the species under con- 

 sideration the foramen is narrow and very much elongated, the length 

 equaling twice the breadth. This conditions the shape of the posterior 

 portion of ischium, which in M. hairdi does not extend far back of the 

 posterior border of obturator foramen, while in Equus the ischium forms 

 a large expanded plate posterior to the obturator foramen. 



The pubis is elongate, flattened from above downward and irregularly 

 triangular in shape. The portion of pubis nearest the acetabulum is 

 almost round in cross-section, while in the horse the corresponding por- 

 tion, as in fact the entire pubis, is very much more flattened. It meets 

 its fellow of the opposite side in the median line forming the anterior 

 part of the symphysis with the bases of the triangles applied together 

 The symphysis is formed by both pubes and ischia conjointly, the former 

 constituting the anterior and larger part while the ischia form the poste- 

 rior part. Fusion of the pubes is so complete that no trace of a suture 

 remains, while the ischia are not anchylosed together. The anterior 

 part of the symphysis is flattened in the form of a large plate, which 

 bears inferiorly in the median line a prominent spine. All the processes 

 for muscular attachments are less strong and rugose than in the horse. 

 The pelvic foramen (or cavity) is longer in proportion to the breadth in 

 M. hairdi than in the horse, being a little longer than broad, while in 

 the latter the pelvic outlet is broader than long. In Mesohippus the 

 length (or vertical height) is about 65 mm. and the breadth 60 mm., 

 while in the horse the reverse condition obtains and we find a length 

 of only 174 mm. as compared with a width of 199 mm.* 



Other measurements of the pelvis are as follows : 



MM. 



1. Extreme length 309 



2. Length of acetabular cavity 26 



3. Length of symphysis 63 



4. Extreme width of ischia 74 



5. "Width at acetabulum 102 



6. From top of angle to outer point of crest 89 



7. From anterior border of acetabulum to point midway 



between angle and crest 74 



Restoration of M. bairdi (Pl. xiii). 



In 1879, Prof. Marsh, f in giving the genealogy of the horse, brought 

 out the fact that the chief moditications through which the horse passes 

 in its evolution are the following : 



1. Progressive increase in the length of teeth and in their complexity, 



* (G/s X 7/b inches) Chauveau loc. cit. ^ Am. Jour. Sci., Vol. xvii, p. 497. 



