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 Cynognathus ; and the Multituberculata and the ex- 
isting Monotremes from some form like Diademodon. 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 Cynoynathus 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 inportant mammalian characteristic ; 
this is brought about by the reduction of the quadrate, which 
latter bone, moreover, acquires in 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- 
