MAY, 1921. AMERICAN MARSUPIAL, CENOLESTES OSGOOD. 99 



20. Astragalus with a large inner malleolar facet nearly per- 

 pendicular to the medial tibial facet; agreeing with saltatorial 

 forms. 



21. Prehallux not present; agreeing with Macropus, Trichosurus, 

 and Phascolomys; disagreeing with Didelphis. 



SKULL. 



Plates XVII, XX. 



General form. The skull of C&nolestes is characterized superficially 

 by the great elongation of the facial region, by the deep and extremely 

 thin- walled braincase, the swollen interorbital and lacrymal regions, the 

 general absence of crests or ridges, the slender zygomata, and by the 

 modifications of the maxillary by which the anterior teeth are at a 

 different level from the posterior. 



The general resemblance to the skull of Perameles was immediately 

 noticed by Thomas, who says (1895, p. 872): "Skull in its general 

 proportions something like that of a Perameles, although thinner and 

 more delicately built,- with a similarly elongated muzzle, smooth and 

 rounded brain-case, and obsolete supraorbital and cranial crest and 

 ridges; the zygomata are, however, so much more boldly expanded as 

 somewhat to spoil the resemblance, which in any case does not apply 

 to details." A still stronger statement of this resemblance to the skull 

 of Perameles might be made and, as appears later, it does apply at least 

 to a certain number of details. The zygomata are more spreading than 

 in some of the peramelids but their sweep is practically identical with 

 that of others (e. g. Isoodon}. Certainly the general form of the skull 

 is nearer that seen in the peramelids than that of any other living mar- 

 supial. The resemblances of general shape to the smaller dasyurids 

 noted by Miss Dederer (1909) seem mostly to be associated merely with 

 small size and are found also in Acr abates and Dromicia. 



The ratio of zygomatic breadth to total length of skull in various 

 marsupials is as follows: Perameles nasuta 39.7, Perameles bougainvittei 

 43.2, C&nolestes 47.7, Isoodon barrowensis 51, Philander 54, Phasco- 

 larctos 54.3, Myrmecobius 56.4, Peramys 56.9, Didelphis 57, Chironecles 

 57.1, Phascologale (Ch&tocercus) cristicauda 63.7, Petaurus australis 66.4, 

 Dactylopsila 67.5, Dasyurus viverrinus 69.7, Trichosurus vulpecula 71.7, 

 Sarcophilus 81.2. In this series, C&nolestes falls with the peramelids. 

 The didelphids occupy a position midway between these dolichocephalic 

 forms and the broader types presented among the phalangers and 

 dasyurids. 



Braincase. The braincase is relatively large and its walls are 



