Sinclair: marsupialia of the santa cruz beds. 349 
for lodgement of the vermis and lateral hemispheres of the cerebellum are 
proportionately as large as in Thylacynus . The olfactory sinuses are 
enormously developed, indicating the possession of keen powers of scent. 
The ascending processes of the premaxillae are shorter than in the other 
Santa Cruz genera. The nasals are greatly expanded posteriorly, occu- 
pying almost the entire width of the interorbital tract and excluding from 
contact the frontals and maxillae. The frontals between the orbits are 
plane in B. tuberata , slightly convex in B. excavata. Postorbital frontal 
processes are entirely wanting and the temporal ridges poorly defined. 
The sagittal crest is lower than in Thylacynus ; its free border is concave 
in profile. The supraoccipital is not exposed on the upper surface of the 
skull, the parietal extending to the margin of the lambdoidal crest as in 
Frothy lacy nits. The orbits are smaller than in Thylacynus and are placed 
farther forward. A large lachrymal tubercle is present on the orbital rim. 
The lachrymal duct opens well within the orbit. A small foramen pierc- 
ing the facial expanse of the lachrymal in B. excavata (PI. XLV, fig. 1) 
may possibly be homologous with the external opening of the lachrymal 
duct in Thylacynus. The zygomatic arches are heavier and more widely 
expanded than in Thylacynus. The postorbital jugal processes are smaller 
and the preglenoid processes larger than in the latter genus. 
The occiput (PI. XLVI, fig. 4) is semicircular in outline and in B. 
excavata does not project posteriorly beyond the condyles, which differ 
from Thylacynus in being wider superiorly and more obliquely placed. 
The areal extent of the mastoid on the lateral border of the occiput is 
proportionately less than in the recent genus. 
The base of the skull is fairly well preserved in the specimen of B. 
excavata (No. 15,120, PI. XLIV). The basioccipital and basisphenoid are 
almost flat. The paroccipital processes are short, resembling Protliyla- 
cynus rather than Thylacynus. The large condyloid foramen is preceded 
by an accessory foramen of approximately the same size. The auditory 
region resembles Prothylacynus in the absence of alisphenoid dilatation, 
but the foramen ovale does not pierce the alisphenoid opposite the glenoid 
cavity, as in the latter genus, the posterior branches of the fifth and 
seventh nerves probably emerging between the alisphenoid and the tym- 
panic. The latter element has been shed, exposing the petrous, which is 
smaller than in Prothylacynus. The palate is without vacuities, but is 
pierced by a number of accessory palatal foramina. The anterior palatine 
