io6 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



enclosing somewhat more of the ventral surface of the presphenoid than 

 usual. It is not emarginate posteriorly and extends slightly beyond the 

 posterior nares. Its ventral surface has a low median ridge but this has 

 no contact with the palatines nor with the maxillaries except anteriorly. 



Tympanic. The tympanic is a simple ring open behind and attached 

 to the alisphenoid bulla by cartilage. It is wider and heavier than in the 

 didelphids but is essentially the same as in Australian polyprotodonts. 



Periotic. The petrous part of the periotic is exposed ventrally as a 

 subtriangular bone between the basioccipital, the tympanic and the 

 alisphenoid bulla. It is thus almost wholly exposed as in polyprotodonts 

 and not partly or almost completely covered by the alisphenoid bulla 

 and the tympanic as in diprotodonts. It is less inflated than in the 

 dasyurids but projects below the surface of the basioccipital slightly 

 more than in the didelphids. Internally it shows the usual openings, 

 including a large floccular fossa. The foramina surrounding it are much 

 as in the other marsupials except for the so-called carotid canal which 

 forms a long open slit between the petrous periotic and the basioccipital 

 which are actually in contact scarcely more than enough to separate 

 this opening from the jugular foramen behind and the entocarotid fora- 

 men in front. Such an opening has not been found in any other mar- 

 supial examined, although in polyprotodonts it may be represented by 

 the small foramen which Gregory (1910, p. 223) has called the posterior 

 carotid foramen. 



The mastoid is relatively larger than in any other form examined and 

 is especially characterized by its great lateral extent. Its occipital sur- 

 face is not more than one-third its lateral and between them there is 

 only a gentle curve or an exceedingly feeble ridge. The lateral surface 

 is slightly convex and meets the squamosal evenly. The short flattened 

 paroccipital process and the mastoid process of the squamosal clasp it 

 on either side making the entire mastoid region relatively smooth and 

 continuous with the general outlines of the skull. This lack of promin- 

 ences is in marked contrast to the condition in the didelphids and is 

 nearer to that of Antechinomys and other primitive dasyurids. A very 

 large mastoid foramen is situated at the upper end of the occipital part 

 of the mastoid separating it from the supraoccipital for a considerable 

 distance. A mastoid foramen occurs in the didelphids but in adults it 

 is very small or practically closed, whereas in C&nolestes it is as large in 

 the adult as in the very small pouch young of Didelphis. 



Hyoid. The hyoid apparatus is of the usual marsupial type. The 

 basihyal is flat and subcircular with its posterior edge free but its 

 anterior and lateral boundaries united with the ceratohyals and thyro- 

 hyals. The ceratohyals are slightly arched forward and only ossified 



