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



exceedingly thin and papery. Viewed in longitudinal section (PI. XVII) 

 its depth is seen to be proportionately greater than in other marsupials. 

 The ratio of the depth to the length measured inside the braincase in 

 several forms is found to be as follows: Ccenolestes 53.9 ; Perameles nasuta 

 48.8; Petaurus australis 47; Myrmecobius 44.4; Trichosurus 42.2; Phas- 

 cologale flavipes 41.7; Marmosa cinerea 41.6; Didelphis virginiana 39.7; 

 Peramys domesticus 38.6. 



The dorsal outline of the braincase is broadly arched, agreeing in 

 this respect with Perameles and with most diprotodonts and contrasting 

 with the didelphids and dasyurids in which the dorsal outline is more 

 nearly straight. The olfactory fossa is large and full, especially in its 

 lateral parts, but it is relatively short antero-posteriorly as compared 

 to didelphids and dasyurids. In these polyprotodonts its median section 

 is subtriangular, often with an acute angle in front, whereas in Ccenolestes 

 and Perameles, and the several diprotodonts examined, it is broadly 

 rounded anteriorly. It is separated from the cerebral fossa by a light 

 ridge, the position of which is plainly indicated externally by the furrow 

 running upward and slightly backward from the ethmoid foramen. 



The cerebral fossa is very full, especially in its lower anterior part 

 where it is extended forward on either side of the median part of the 

 cribriform plate. The floor of the braincase is thus wider and flatter 

 anteriorly than in other marsupials and it meets the cribriform plate 

 more abruptly. This is in the region occupied by the tuberculum olfac- 

 torium which, as elsewhere noted, is unusually large (see p. 159). In 

 front of the occipito-sphenoid suture is a shallow pituitary fossa. The 

 sphenoidal fissure and the foramen rotundum open posteriorly on the 

 floor of the braincase and have no lateral exposure like that in other 

 marsupials. Externally the sphenoidal fissure and the foramen rotundum 

 have what is practically a common opening, the bony septum between 

 them being quite internal and not visible in lateral view. In this, the 

 condition seems nearer that of the diprotodonts than of the polyproto- 

 donts in which the foramen rotundum opens decidedly posterior to the 

 sphenoidal fissure with the septum between them broadly exposed in 

 lateral view. 



The cerebellar fossa is smooth and without obvious division into 

 lateral and median parts. It is separated from the cerebral fossa by an 

 exceedingly slight tentorial ridge. The outline of the cribriform plate 

 as seen in section is more similar to that of Perameles than to that of any 

 other form examined. It is more nearly vertical than in polyprotodonts 

 generally and more abruptly divided between its upright and its posterior 

 basal extension in the presphenoid region. This posterior extension is 

 broader and higher than in any other marsupial examined. 



