172 • PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Tapir, Stag, and placental Herhivora, If it were objected that the 

 first and second of the five grinders in Zygomaturus or Notoiheriwm 

 may be deciduous teeth, destined to be succeeded by vertical succes- 

 sors or premolars, it may be replied, that no instance of the retention 

 of deciduous molars with a last molar in place, and so worn as is the 

 fifth grinder of Zygomaturtis, has been observed. 



A view of the upper molar series of a Tapir, soon after the last 

 molar has come into place, is given in Plate VIII. fig. 4, to illustrate 

 this difference from the state of the molar series in the Australian 

 fossil (Plate YIII. fig. 5), in which the tooth, marked d^, the fourth 

 from the back end of the series, is much more worn than mi, the 

 third from the same end. This state of the dentition determines 

 the marsupiaHty of this huge and most strange extinct quadruped as 

 decisively as would the marsupial bones, had the entire pelvis been 

 found. 



All the cranial characters elucidating marsupial affinity concur 

 with the dental ones in establishing it. 



The brain was very small ; its proper case makes no swelling from 

 the inner wall of the temporal fossa (Plate YII. fig. 3, t) as it does in 

 the Ox, Horse, Tapir, or other placental Herbivores of like size with 

 the fossil. Equally indicative of the low condition of cerebral de- 

 velopment is the inclination of the plane of the occiput from the 

 condyles upward and forward, as shown in fig. 2, Plate YII.; 

 and in fig. 3, taken looking directly upon the upper surface of 

 the skull, in which the whole sloping occiput comes into view, 

 divided from the upper surface by a super-occipital ridge which 

 describes an open angle with the apex forwards. The constricted 

 part of the cranium in advance of this ridge, and opposite the 

 middle of the temporal fossae*, marks the anterior boundary of the 

 cranial cavity : the part / in advance, which gradually expands, 

 answers to that part in the Kangaroo and Wombat which is occupied 

 by the extensive cellular diploe, in communication with the nasal 

 cavityt ; such expansion of the pneumatic frontal bone is also found 

 in the Phascolomys latifrons. The zygomatic arches, from their 

 depth, thickness, outward span, and descending process (^6. fig. 2, 

 z) present one of the most marked and peculiar features of the fossil 

 skull from Darling Downs ; the extent of the fossae which they cir- 

 cumscribe, and their proportions to the rest of the skull, recall the 

 features of that of the Elephantine Seal (Cysto^Jiora jprohoscidid) ; 

 but the form and direction of the descending process, z, show this arch 

 to be essentially an exaggeration of that of the Koala (Plate YIII. 

 fig. 2) and Kangaroo. The descending process is, however, re- 

 latively larger and longer : in this respect it resembles the same 

 part in the skull of the Diprotodon. 



The power of the muscles of the mandible both for biting and 

 chewing must have been enormous, and indicates some peculiar 

 quality of resistance in the alimentary substances to be ground down. 

 The grip of the strong and long anterior incisors, brought by the 



* See ' Osteology of the Marsupiaha,' loc. cit., pp. 380, 386. 



t lb. Part II. Trans. Zool. Soc, vol. iii. part iv, p. 303, plate 37. figs. 4 and 5. 



