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 Phascolotheriwm and Amphilestes, as well as 
Triconodon and Spalacotherium. Finally we have (5) the Tritu- 
berculata (or Insectivora Primitiva) with the genera Amphitheriuwm, 
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 1s only known by lower jaws implying an 
animal of the size of a Rat or rather smaller. The jaws have in 
front a large incisor which looks Rodent-lke, 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 rim is beset by tubercles. Other Jurassic genera. 
of Multituberculates are Solodon, Allodon, and Stereognathus. 
All of these possess the same multituberculate molars. 
Of the Triconodonta the type-genus is 7riconodon. This genus 
is better known than most Jurassic mamials, 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. 
