292 ORDERS OF MAMMALIA. 



swims the Port- Jackson Shark (Cestracion Philippi) ; whilst 

 the MoUuscau genus Trigonia is nowadays exclusively con- 

 fined to the Australian coasts. In England at the time of 

 the deposition of the Stonesfield Slate, we must have had a 

 fauna and flora very closely resembling what we now see in 

 Australia. The small Marsupials Anipliitlierium and Fhas- 

 colutherkvm prove that the Mammals were the same in order ; 

 cones of Araucarian pines, with tree-ferns and fronds of 

 Cycads, occur throughout the Oolitic series ; spine-bearing 

 fishes, like the Port-Jackson Shark, are abundantly repre- 

 sented by genera such as Acrodus and Strophodus ; and 

 lastly, the genus Trigonia, now exclusively Australian, is 

 represented in the Stonesfield Slate by species which differ 

 little from those now existing. 



Another singular point of resemblance is established by 

 the occurrence in the rivers of Queensland of the " Barra- 

 munda," wdiich is referred to the genus Ceratodus — a genus 

 which, though pre-eminently Triassic, nevertheless extended 

 its range into the Jurassic period. 



Towards the close of the Oolitic period, in the Middle 

 Purbeck beds, we have evidence of a number of small Mam- 

 mals, all of which are probably referable to the Marsvpialia, 

 and all of which, except Plagicmlax, are Polyprotodont. 

 Fourteen species are known, all of small size, the largest 

 being no bigger than a polecat or hedgehog. The genera 

 to which these little quadrupeds have been referred are 

 Plagiaulax, Spcdacothermm, Triconodon, and Galestes. The 

 first of these — viz., Plagiaulax (figs. 604 and 605) — is be- 

 lieved to be most nearly allied to the living Kangaroo-rats 

 {Hypsiprymnus) of Australia ; and it is held by good author- 

 ities to have been phytophagous, as are its living relatives. 

 Prof. Owen, on the other hand, believes that Plagiaulax was 

 carnivorous. The chief feature in the dentition of Plagiaulax 

 is found in the fact that the premolars are marked on 

 the exterior of their crowns with seven conspicuous grooves 

 (fig. 605, A and b), which entirely resemble the grooves in 

 the large first pmemolar of the living Hypsipnjmmis, except 

 that they are diagonal and not vertical in direction. The 

 lower incisors have an upward curvature, which is not the 



