1898.] 



159 



[Farr. 



Fis. 4. 



Pelvis of M. bairdi, ^. 



The Pelvis (No. 11376). 



The pelvis is equine in all its characters and very much like that of the 

 modern horse with some characteristic points of difference. The speci- 

 men described below is the first pelvis of Mesoliippus bairdi that has ever 

 been found showing all the characters, being almost perfect. See Fig. 

 4, and Plate xiii. It was discovered by 

 Mr. J. W. Gidley during the past sum- 

 mer in the lower Oreodon beds. 



The most striking difference between 

 the pelvis of M. bairdi and that of the 

 horse is that the former is narrower in 

 proportion to its length than that of 

 Equus. 



The great breadth of the pelvis an- 

 teriorly in the latter is owing to the very 

 great lateral expansion of the ilia, while 

 in the earlier genus they are propor- 

 tionately less widely expanded. The ilia 

 directly in front of the acetabulum are 

 slender in their proportions and expand 

 more gradually than in the horse, so that 

 they are longer in proportion, to their 

 width than in the latter. The bone is 

 widely expanded superiorly and the angle above the point of articulation 

 of the ilium with the sacrum curves upward and outward, and the free 

 end is thickened and somewhat rugose. This upward and outward ex- 

 pansion of angle makes the external border of superior aspect of the 

 ilium concave. The crest is more slender and elongate comparatively 

 than in Equus and is strongly everted. The border of the ilium between 

 the angle and the crest is very thin and strongly concave. The whole 

 anterior expanded portion is thin except along the outer or lower 

 border. The posterior border of the angle above the point of articula- 

 tion of the sacrum is also slightly thickened. The sacral border of the 

 ilium is large and extends high above the articular facet for the sacral 

 vertebrae forming the angle. The ilia as well as the long axis of pelvis 

 are directed downward at an angle from the vertebral column. The 

 acetabulum is an elongate oval in shape and its borders are elevated and 

 well-defined. The border is incomplete below owing to the encroach- 

 ment of the pit for the ligamentum teres on the acetabular fossa. This 

 is less emphasized, however, than in the horse. The pit for the liga- 

 mentum teres is quite deep. 



The ischium is straight and on a line with the long axis of the ilium. 

 The bone curves outwardly posteriorly, but does not curve upward as in 

 the horse. The posterior border is expanded and thickened outwardly 

 where it ends in a stout process, the tuberosity of the ischium. The in- 

 ternal border posteriorly is deflected towards the median line and meets 



