PERISSODACTYLA. 
In this order the number of the foramina ranges from very few to many. 
Fra. 3.—A phelops meqalodus Cope, Loup Fork of Colorado; one-sixth natural 
size; showing postparietal foramen. 
Rhinocerus, Aphelops ; postparietal only. 
Fic, 4. ° 3 Fie. 5. 
skulls of Aphelops megalodus (Fig. 4) and A, fossiger (Fig. 5) from behind; one- 
sixth natural size; showing absence of mastoid foramen. 
Tapirus ; postparietal and a huge mastoid. 
Anchitherium, Hippotheriwm, Protohippus and Equus; postparietal, 
postsquamosal, postglenoid and supraglenoid. In the last three genera 
the vessel issuing from the postsquamosal, grooves the petrous bone, leav- 
ing it ata point near that usually occupied by the mastoid foramen, In 
the second and last genera, and probably in the third, the sinous canal is 
protected by a bony crest in front, throughout its entire length. 
