SINUS PARANASALES EARLY TERTIARY MAMMALS 139 



larged to diameters of three to four millimeters. Perhaps in 

 some of the oreodonts, such as Promerycochoerus, which had a 

 much more highly developed sagittal crest, the sagittal saccu- 

 lations were more highly developed. The sinuses, as they are 

 developed in the present specimen, more nearly resemble those 

 of the sheep than those of the pig, with which the oredonts have 



Fig. 4 Outline of the skull of Alerycochoerus (After Matthew) superimposed 

 on the drawings of the casts of the accessory nasal sinuses, to show the loca- 

 tion of these cavities in the head. X 3. 



some affinity. A skull showing the accessory sinuses of the 

 sheep was used for comparison with the oreodont. 



The group of small cavities (figs. 1, 2 and 4) just anterior 

 to the brain, are, doubtless, all divisions of the frontal sinus. In 

 the frontal region of a calf, Mihalkovics ('99; Taf. 5, fig. 24) 

 has figured a sacculation of the frontal sinus, which in this fossil 

 form has taken the shape of separate cavities. This condition 

 of the sinuses, however, is, in the human skull, subject to such 



