DISTRIBUTION OF MAMMALIA IN TIME. 



641 



tenanted by Marsupials ; upon its land-surface flourish Arau- 

 caricz and Cycadaceous plants, and in its seas swims the Port 

 Jackson Shark (Cestracion Philippi)', whilst the Molluscan 

 genus Trigonia is nowadays exclusively confined to the Aus- 

 tralian 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, Amphitherium and Phascolotherium, 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 represented by genera such as Acrodus 

 and Strophodus ; and, lastly, the genus Trigonia, now exclu- 

 sively Australian, is represented in the Stonesfield Slate by 

 species which differ little from those now existing. 



Fig. 273. Oolitic Mammals, natural size, i . Lower jaw and teeth of Phascolotheritnn : 

 2. of Triconodon ; 3. of A tnphitfteriutn ; 4. of Plagiaulax. 



In the middle Purbeck beds (Upper Oolite), where fourteen 

 species of Mammals are known to exist, it is probable that all 

 were Marsupial. All the Purbeck Mammalia were of small 

 size, the largest being no bigger than a pole-cat or hedgehog. 

 They form the genera Plagiaulax, Triconodon, and Galestes, of 

 which Plagiatilax is believed to be most nearly allied to the 

 living Kangaroo-rat (Hypsiprymnus] of Australia. 



In the Tertiary series of rocks Marsupials are of rare occur- 

 rence ; but an Opossum, closely allied to the existing American 

 forms, has been discovered in the Eocene rocks of France 

 (Gypseous series of Montmartre), and has been named the 

 Didelphys gypsorum. 



The next occurrence of Marsupials is in the later Tertiary 

 (Pliocene) and in the Post-tertiary epoch ; and here they are 

 represented by some very remarkable forms. The remains in 

 question have been found in the bone-caves of Australia the 

 country in which Marsupials now abound above every other 



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