iv PLAGIAULAX 99 



Theromorphous group). (2) The Triconodonta, which were 

 Marsupials, though in all probability with a complete succession 

 of teeth and with an allantoic placentation. This group will 

 include the genera Phascolotherium and Amphilestes, as well as 

 Triconodon and Spalacotherium. Finally we have (3) the Tritu- 

 berculata (or Insectivora Primitiva) with the genera Amphitherium, 

 Peramus, Amblotherium, Stylacodon, and Dryolestes. 



We shall take these three groups in order. The Multituberculata 

 have already been to some extent defined, if such a word can be 

 used to express the summation of the very scanty information at 

 our disposal. Of this group, Plagiaulax is a genus which occurs in 

 the Purbeck beds ; it is only known by lower jaws implying an 

 animal of the size of a Eat or rather smaller. The jaws have in 

 fronb a large incisor which looks Rodent-like, and also like those of 

 the Diprotodont Marsupials ; but it is held that these teeth did not 

 grow from persistent pulps, and there is in any case no anterior 

 thickened coating of enamel. Canines are absent ; the diastema 

 is followed by four premolars increasing progressively in size and 

 possessing somewhat complicated grinding surfaces. These surfaces 

 are formed by several obliquely-set ridges. The succeeding teeth 

 are termed molars on account of their difference in structure, and 

 there are but two of them on each side. The molars are of a 

 pattern common in the Multituberculata ; the centre is hollowed, 

 and the raised rirn is beset by tubercles. Other Jurassic genera 

 of Multituberculates are Bolodon, Allodon, and Stereognathus. 

 All of these possess the same multituberculate molars. 



Of the Triconodonta the type-genus is Triconodon. This genus 

 is better known than most Jurassic mammals, since both the upper 

 and the lower dentition have been described. It appears to have 

 possessed the typical Eutherian dentition of forty-four teeth, to 

 which a fourth molar is added in some species. The great differ- 

 ence between the molars and premolars argues a complete tooth- 

 change. The genus is American as well as European. 



Spalacotherium has more molars, five or six. 



Phascolotherium bucklandi, on the other hand, is a much 

 older type in the form of its teeth. There are, however, not so 

 many of them as in Amphitherium ; Phascolotherium has but two 

 premolars and five molars, making a total of forty-eight teeth. 

 The teeth are of the triconodont form, the three cusps being in 

 line, and the middle one the largest. 



