Ill SKULL OF CYNOGNATHUS 93 



and in Diademodon for instance. This incidentally lends some 

 support to the idea that the Mammalia have been evolved from 

 two sources, a way of looking at the origin of the group that 

 will coincide with the views of some authors like the late Dr. 

 Mivart, and will at the same time reconcile the trituberculists 

 and the multituberculists. For we should then assume that 

 the Eutheria and Triconodontia had originated from some such 

 form as Cynognatlms ; and the ]\Iultituberculata and the ex- 

 isting Monotremes from some form like BicuUmodon. It is not 

 of great use to point out that Diademodon is really of the tri- 

 tuberculate pattern, because in its molars, though multituberculate, 

 the trituberculate main cones can be recognised ; for that state 

 of affairs could just as well have been brought about by a 

 reduction from the multituberculate type. The skull of these 

 Theriodonts shows some well-marked approximations to the 

 mammalian type. There is in the first place a commencing 

 consolidation and reduction of the individual bones, which is so 

 distinguishing a feature of the mammalian skull as opposed to the 

 skull of lower vertebrates. In Gynognathus the postorbital is 

 fused with the jugal, and the supratemporal with the squamosal, 

 forming apparently one bone. In the lower jaw the splenial is 

 often reduced to the thinness of paper, thus indicating a com- 

 mencing disappearance. In many Theromorpha the squamosal 

 shares largely in the formation of the articular facet for the 

 lower jaw, obviously an important mammalian characteristic ; 

 this is brought about by the reduction of the quadrate, which 

 latter bone, moreover, acquires iu certain particulars the appear- 

 ance of the mammalian malleus, with which it is, according 

 to many, homologous. But this subject has been already dealt 

 with on page 26. A very ]pronounced likeness to the mam- 

 malian skull is that there are two occipital condyles. That 

 this has been brought about by the further development of 

 a tripartite condyle such as occurs in tortoises, and that by 

 the suppression of the basi - occipital part, does not affect the 

 resemblance to the mammalian skull ; in fact it explains the 

 origin of two condyles from the typical reptilian single con- 

 dyle, and disposes of the necessity for believing, with Huxley 

 and others, the Amphibia to be on the main line of mam- 

 malian evolution on account of their two condyles. The 

 general aspect of the skull in Cynognathus has been com- 



