1901.] AFFINITIES OF TJDENODON. 187 



not apply to those cases where two elements distinct in early life 

 coalesce or anchylose as development advances, but it appears to 

 apply to membrane-bones as well as cai'tilage. 



In Udenodon the small size of the intermedium seems to 

 indicate that it is on the point of disappearing, and in mammals 

 it is apparently quite lost. The centrale, though as displayed it 

 is small, occupies the position of the navicular in the mammal, 

 which has generally been regarded as the centrale of the 

 tarsus. 



The digits and the phalanges agree in number with those of the 

 Mammalia. 



Conclusion, 



Prom the structure of the skull and other parts of the skeleton, 

 Udenodon is very manifestly closely related to both the Theriodonts 

 and to the Mammals, and though the higher Theriodonts are too 

 specialized to have been the ancestors of the Anomodonts, the 

 lower Theriodonts may very well have been the ancestors not only 

 of the Anomodonts but of the Mammals. The lower Theriodonts 

 may have sprung from forms allied to Pareiasaurus with the 

 temporal region completely roofed ; and it seems probable that 

 the bones forming the temporal roof have been reduced in number 

 before the first formation of the fenestra. From the same line by 

 which the Theriodonts have arisen, it is probable that the Plesio- 

 saurs have sprung as an offshoot arising early and becoming 

 greatly specialized, and it is also possible that the Cheiouians have 

 branched off from near the same point. 



All the other reptilian groups appear to have arisen by a different 

 branch or branches from the primitive forms. Prom forms with 

 the temporal region completely roofed as in ParioticJius, it is 

 probable that a group arose with the roof fenestrated at first in 

 the supralateral region, and then later in the infralateral region 

 as well, and giving rise to a group of which PalceoJiatteria may 

 be regarded as the type. Prom this group it is highly probable 

 that the Pelycosauria have arisen by one branch, the Crocodilia 

 by another, the Dinosauria, and probably the Ornithosauria, by a 

 third ; while a fourth branch has given rise to the Ehyncho- 

 cephalia and the Squamata. The Ichthyosauria appear to have 

 either sprung directly from the primitive group, or to have 

 branched off very early from the PcdceoTiatteria stem. All the 

 orders along this main branch have the palate directly derived 

 from a Sphenodon-WkQ type, and in most instances the modification 

 from the type is sHght. 



What appear to be the mutual relationships of the principal 

 Orders are best seea when an endeavour is made to arrange them 

 phylogenetically, thus : — 



