1910.] Tritylodon, Microlestes. 167 



would have been impossible if the articulation had been by way of the quad- 

 rate and articidar. Accordingly Broom concludes (1907, p. 7) that the jaw 

 worked by means of a mandibulo-sciuamosal articulation and that therefore 

 TritijJodon was a mammal. 



If Tritijlodon is a IVIuItitubercuIate (and no contradictory evidence 

 appears at present) then it must go with the Multituberculata into the 

 Marsupialia. The existence of a highly specialized Marsupial in the Lower 

 Jurassic (Broom) Avould indicate a still earlier origin of the Marsupial stem. 

 At the same time the profile of the Tritylodon skull is rather suggestive of 

 what may be conceived to have been the ancestral Monotreme type, i. e., 

 a long snout, long diastema, small anteriorly placed orbit, narrow frontals, 

 multicuspid molars. It is also significant that in OrnitJiorhynckus the 

 glenoid fossa is extended antero-posteriorly and that the jaw can move 

 backward and forward to some extent. 



Another early ]Multituberculate is Microlestes (figured by Osborn, 1888, 

 p. 214), and known from three species founded upon isolated lower teeth 

 from the Upper Triassic, or Rhsetic, of Germany and England. The 

 evidence tending to connect Microlestes with the Multituberculates is briefly 

 as follows: 



(1) The Microlestes antiquus molar while more elongate and compressed, 

 resembles the second lower molar of Plagiaulax minor, a Multituberculate 

 of the Upper Jurassic, in possessing a high antero-internal cusp on the raised 

 internal border and a row of small external cusps. In both genera the molars 

 have a median longitudinal basin and two roots. 



(2) The lower molar called "Microlestes" moorei, comes from a forma- 

 tion from which only one other mammalian fossil is known: viz., the type of 

 Hypsiprymnopsis rhcetiens Dawkins. This is a grooved lower premolar 

 of IMultituberculate type. 



The order Multituberculata, being thus already well differentiated in 

 the Upper Triassic, is known also in the Jurassic of England and the conti- 

 nent and persisted into the Upper Cretaceous and Basal Eocene of Europe 

 and North America. Its reported presence in the Eocene of South America 

 appears somewhat doubtful (p. 211). 



Origin of the Multituberculate Molar. 



The order Multituberculata had a very long history stretching from the 

 Upper Triassic into the Basal Eocene, during which it presents several 

 modifications of a very well defined molar type. 



It has been suggested that the IMultituberculate molar may have been 

 derived from the tritubercular type, and certain supposedly analogous cases 



