140 ROY L. MOODIE 



an enormous range of variation that it cannot be said at present 

 whether the small sinuses in Merycochoerus are primitive or 

 are simply peculiar to this individual form. An extended study 

 of casts of these animals would settle this point. None of the 

 cavities in the fossil seem to be due to infoldings of the ethmo- 

 turbinal, but are either divisions of the frontal or maxillary 

 sinuses. The smaller posterior, or frontal sinuses, are sepa- 

 rated from each other by rather thick partitions of bone (alae 

 ossi frontali). It is quite evident that the large cavities just 

 anterior to the small posterior ones, are sacculations of the 

 maxillary sinus, which have been named (fig. 2) the sinus 

 maxillaris superior, as in the sheep and horse. The relatively 

 great distance, thirty millimeters, between the most anterior 

 one of the sinus maxillaris superior and the base of the sinus 

 maxillaris inferior is traversed by a canal which is possibly 

 homologous to the ostium maxillare. This condition is particu- 

 larly parallel in the sheep, but whether it is proper to speak of 

 this opening as the ostium maxillare, or not, is uncertain. 



The divisions of the frontal sinus are small, slender, and 

 irregularly developed (figs. 1 and 2), exhibiting characters which 

 are, so far as I am aware, unparalleled in modern mammals. 

 The descending wings of the frontal bone, which separate the 

 sinus frontales from the sinus maxillaris superior, are relatively 

 thick as in the sheep, and from the anterior surfaces of these 

 alae in the oreodont, as in the sheep, doubtless sprang the 

 ethmoturbinals. 



There are five sacculations of the sinus maxillaris superior. 

 They are much larger than the sinus frontalis, and their sur- 

 faces are richly supplied with blood vessels (fig. 1), which are, 

 I suppose, branches of the A. ethmoidalis anterior. The most 

 posterior division of this group of sinuses, is quite slender supe- 

 riorly and deep posteriorly, forming a cavity which was flattened 

 by the anterior part of the jugal arch. A lateral inferior divi- 

 sion is pea-shaped. It is quite prominent and projects sharply 

 into the maxillary bone. 



The sinus maxillaris inferior is enormously deevloped, and in 

 the cast, takes the form of two backwardly projecting horns. 



