302 OUTLINES OF EVOLUTIONARY BIOLOGY 



yielded a fragment of a jaw, containing three small multituber- 

 culate grinding teeth, to which the name Stereognathus has been 

 given, and one of the Middle Purbeck beds of Swanage (Upper 

 Jurassic) has contributed the well known jaws of Plagiaulax 

 (Fig. 148). Remains of closely related forms, supposed to have 

 belonged to the same family (Plagiaulacidae), have been discovered 

 in Upper Jurassic and Upper Cretaceous formations of North 

 America, and the family appears to have survived, both in Europe 

 and North America, into early Tertiary times. 



The Plagiaulacidae and related forms have been grouped together 

 in the extinct order Multituberculata, which Dr. Smith Wood- 

 ward places provisionally amongst the Prototheria, side by side 

 with the surviving order Monotremata, no remains of which are 

 p known to occur before Tertiary 



times. It is possible, however, 

 2 ^MS^SSVSSSU ' > ^S that the Multituberculata may be 



metatherian rather than proto- 



therian. 



The remains of undoubted Meta- 



theria (Marsupialia) first occur, so 



FIG. 148. Mandible of Plagiaulax , 



minor, x 4. (From Smith far as we yet know, in the same 

 Woodward's "Vertebrate beds as the earliest Multituber- 

 PalsBontolo^v " al'tsr Fal- j P 



con.6r.) o 



the enigmatical Tritylodon and 



Microlestes). Mandibles of Phascolotherium (Fig. 149) and 

 Amphitherium have been found in the Stonesfield slate, while 

 Triconodon and Spalacotherium are similarly represented in the 

 mammal bed at Durdlestone Bay near Swanage. 



Throughout the whole of the Secondary period the mammals 

 remained of insignificant size, and in a more or less primitive 

 condition, such as is represented at the present day by the 

 surviving monotremes and marsupials. The typical placental 

 mammals (Eutheria) are not known to us from formations of 

 earlier date than the Eocene. Then, all at once, they seem to have 

 branched out in every direction and taken possession of land, sea 

 and air just as the reptiles had done before them ; whales replacing 

 the Ichthyosauria and Plesiosauria, various groups of land 

 mammals replacing the Theromorpha and Dinosauria, and bats 

 sharing with the birds the kingdom of the air which had formerly 

 belonged to the pterodactyls. 



As in the case of their amphibian and reptilian predecessors, 



