322 MEMOIRS OF THE CARNEGIE MUSEUM. 
and triangular in cross-section behind the posterior spine which is not high. The 
ilium expands somewhat posteriorly to form the anterior walls of the acetabulum. 
The os pubis is relatively small. In cross-section immediately in advance of 
the anterior extremity of the thyroid foramen it is subtriangular, the base of the 
triangle being represented by the anterior free surface. From this point the 
bone expands broadly inwardly, becoming somewhat pyramidal in form, the base 
of the pyramid being located at the symphysis and continued backward as the 
ramus forming in part the inner boundary of the thyroid foramen to the point 
where codéssification with the ramus of the ischium takes place. The os pubis 
expands externally and enters into the composition of the acetabulum to the 
extent of about one-fifth, including the lower half of the cotyloid notch, as is shown 
in the case of the left innominate bone of specimen No. 1705, in which the sutures 
in the acetabulum are visible. The acetabulum is large and deep. The pit for 
the round ligament is large and located in the posterior portion of the cavity. 
The ischium is intermediate in size between the smaller pubis and the vastly 
larger ilium. In M. petersoni the bone is relatively shorter than in M. elatus and 
is not strongly everted at its extremity as in the latter species. In the portion of 
the bone found, the external lateral margin of the thyroid foramen is triangular in 
cross-section, passing insensibly into the broad plate which forms the posterior 
portion of the bone, which is thickest laterally and terminates in the strong, rough- 
ened tuberosity. The inner margin of the bone is thin on its edge and produced 
forward in the ramus, which codssifies with the ramus of the pubis to form the 
inner margin of the thyroid foramen. The anterior part of the ischium enters 
into the composition of the acetabulum for about one-fourth of its surface. 
The thyroid foramen is elongated, oval, with the longer axis parallel to the line 
of the symphysis. 
The pelvis when compared with that of other Perissodactyls is seen to be 
relatively longer and narrower than is usually the case in this suborder, and especi- 
ally when compared with the Titanotheres of the Oligocene. This elongation is 
chiefly brought about by the extension behind the acetabulum of the ischium and 
pubis to which Peterson called attention (l. c., p. 748). The gluteal surface of the 
ilium, when compared with the same surface in other genera, is relatively small. 
It is more everted than in Titanotherium, and more closely resembles the corre- 
sponding part in such forms as Cenopus tridactylus, Trigonias osborni, and the 
modern horse, except that the supra-iliac border is filled out more roundly than in 
the latter genera. The supra-iliac border rises high above the pleurapophyses of 
the sacrum, very much as in Trigonias, Diceratherium, and other Perissodactyla. 
