270 the merycoidodontim: 



The divisions of the frontal sinus are small, slender, and irregularly developed, 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 frontalis from the sinus maxillaris superior, are relatively thick as in the sheep, and from the 

 anterior surfaces of these alas 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 surfaces are richly supplied with blood vessels, which are, I suppose, branches of the A. ethmoidalis 

 anterior. The most posterior division of this group of sinuses, is quite slender superiorly and deep posteriorly, 

 forming a cavity which was flattened by the anterior part of the jugal arch. A lateral inferior division is pea- 

 shaped. It is quite prominent and projects sharply into the maxillary bone. 



The sinus maxillaris inferior is enormously developed, . . . and takes the form of two backwardly projecting 

 horns. These sinuses occupied the entire cavity of the maxillary bone, as in the sheep, and projected posteriorly 

 into the zygomatic arch, to a point below the orbit. The maxillary sinuses are not so many nor so complex as in 

 the sheep, but the posterior division of the inferior group extends much farther into the zygomatic arch. There 

 are several minor sacculations of the sinus maxillaris inferior, one below the horn-like projection. Anteriorly 

 there is a rather large accessory sinus. Leading backward from this cavity is a canal which is possibly homologous 

 to the ostium accessorium of human anatomy. The accessory sinus is not very prominent laterally, but has a large 

 expanse anteroposteriorly, extending beyond the premaxillo-maxillary suture. 



The two canals, which have been designated ostium maxillare and ostium accessorium, connect the two great 

 divisions of the sinus maxillaris (inferior and superior) with the sinus frontalis, and for this reason I have sup- 

 posed they have been properly designated. The surfaces of all the casts are richly supplied with blood vessels. 



In a cast of the frontal and other sinuses of Merycoidodon gracilis, the topography of this area 

 is shown to follow the pattern of Merycochcerus by Moodie (1922, pp. 366-367), who described it 

 in part as follows: 



The casts of the frontal sinuses are mostly broken, but the portions remaining indicate a multiple nature for 

 this sinus in Oreodon gracilis, as it is found to be in Merycochcerus. Three small sacculations of the sinus maxil- 

 laris superior come into intimate relation with the olfactory bulb, being separated from actual contact by the 

 paper-like alas of the frontal bone. The smooth surfaces of all the casts of the sinus maxillaris superior, of which 

 there are five on each side, are covered with a rich network of rami of the arteria ethmoidalis supplying the 

 mucosa sinus paranasali. Judging from the condition in Merycochcerus, the paired median sacculations of the 

 sinus frontalis lay immediately superior to the olfactory bulbs, and we have these bulbs exposed in Oreodon 

 gracilis because the sinuses have been broken away. 



I believe Moodie is entirely correct when he observed that "the origin of the sinus paranasales 

 is to be found, not in the early mammals, but in their ancestors, and probably their remote ancestors." 



