MAY, 1921. AMERICAN MARSUPIAL, CENOLESTES OSGOOD. 105 



magnum. Its ventral surface is smooth or even slightly concave medially 

 without suggestion of a central ridge. This constitutes another slight 

 and perhaps unimportant, but still practically exclusive, resemblance to 

 the peramelids. 



Sphenoid bones. The basisphenoid is decidedly concave on its 

 exposed surface but has no sharp lateral ridges. It is perforated as usual 

 by the transverse canal and the entocarotid foramen. The suture 

 between it and the basioccipital is closed in adults although usually 

 indicated by a faint transverse ridge. On its superior or cranial surface 

 it is excavated posteriorly for the reception of the hypophysis but there 

 is no distinct sella turcica nor clinoid process. 



The presphenoid is only slightly exposed ventrally, being largely 

 inclosed by the thin irregular extensions of the palatine, vomer, and 

 pterygoids. The suture between it and the basisphenoid, usually so 

 distinctly open, is but faintly indicated. Within, the vertical part of the 

 presphenoid is slightly thickened basally but soon becomes exceedingly 

 thin. The orbitosphenoid occupies a small area between the lacrymal 

 and the sphenoidal fissure, its exact boundaries being uncertain. The 

 alisphenoids show no peculiarities beyond those connected with the 

 unusual fullness of the braincase in the region covered by them. As 

 stated elsewhere, the sphenoidal fissure and foramen rotundum are 

 practically confluent just before reaching the exterior but those of the 

 two sides of the skull are more distinctly separated from each other 

 than in any other form examined. The alisphenoid bulla forms a 

 simple subhemispherical capsule similar to that of the didelphids and 

 dasyurids. 



Palatine. Owing to the very large posterior vacuities the palatine 

 bones are relatively small. In the median line they send a pair of slender 

 processes forward to the plane of the middle of the second molar where 

 they meet similar processes from the maxillaries. Posteriorly they are 

 bounded by a slight transverse ridge which is perforated on each side 

 by a small postero-lateral foramen. In close proximity to this foramen is 

 the slightly larger spheno-palatine foramen piercing the upright plate. 

 This plate forms the side walls of the posterior part of the nasal passage, 

 but anterior to the spheno-palatine foramen the walls become dis- 

 continuous since the turbinals are fenestrated and exposed much farther 

 caudad than usual and the narrow vomer does not reach the side walls. 

 Posteriorly the palatines form part of the sloping ridge which supports 

 the pterygoids. The pterygoid processes are essentially similar to those 

 of polyprotodonts although they take the form of slender rounded 

 prongs rather than flattened subtriangular plates. 



Vomer. The vomer is narrow in front and expanded posteriorly 



