342 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



length of the cranium. The brain case is small in the Santa Cruz genera 

 and greatly constricted postorbitally, and the orbits are placed much 

 farther forward than in the Dasyuridce, opossums, or Thylacynus. In the 

 latter genus (PI. LXV, fig. i), the capacity of the brain case has increased 

 considerably, with a corresponding expansion of the postorbital region, 

 both of which, together with the posterior shifting of the orbit, may be 

 regarded as progressive characters. In the extinct members of the family 

 the palate lacks the vacuities present in all existing carnivorous marsupials, 

 but is perforated by a number of accessory palatine foramina. Between 

 the molars, the margin of the palate is depressed into deep hemispherical 

 fossae for reception of the tips of the lower teeth when the mouth is 

 closed. The jugal arches are robust and rather broadly expanded, and 

 the sagittal and lambdoidal crests well marked, but not very high. In the 

 Santa Cruz forms, the occiput is semicircular in outline, in contrast with 

 its triangular shape in the dasyures, SarcopJiilus and Thylacynus. The 

 lachrymal canal opens well within the orbital rim. In the majority of 

 living marsupials, the opening of the lachrymal duct is placed either on 

 or external to the orbital rim. Thylacynus is transitional between these 

 two types of structure in that it possesses a double lachrymal perforation, 

 one branch of the canal opening without and the other within the orbit. 



The number and position of the cranial foramina in the existing and 

 extinct members of the family are practically the same with one important 

 exception, namely, that in Thylacynus the basisphenoid is perforated by 

 two large foramina, as in DidelpJiys (cf. PI. LXV, fig. \d] whereas in the 

 Patagonian forms there is but is but a single perforation. 



Borhycena and Prothylacynus resemble SarcopJiilus in the fusion of the 

 mandibular symphysis. In the remaining genera the symphysial union 

 is ligamentous. 



2. The molars of the Santa Cruz genera are of the same type as in 

 Thylacynus, differing principally in the greater reduction of M A , the loss 

 of all the styloid cusps, except the antero-external, and the character of 

 the heel of the last lower molar, which may be either small and conical, 

 basin-shaped, or bicuspidate. The premolars are unreduced in number 

 and usually increase in size posteriorly in both series. The canines are 

 long, sharply pointed and slightly curved in the smaller genera. In Bor- 

 hyczna the fang is swollen and the point short and blunt. The incisors 

 in Borhyczna are reduced to , an exceptional formula among marsupials 



