70 MR. D. JI. S. WATSON OX THE 



Udaj-^hosau7-HS, they show evolutionaiy changes which are in the 

 main in the same direction as those vs'hich lead from Varanoscmrus 

 to Diademodon, coupled, of course, with many individual speciali- 

 zations. Although it is not essential for the purpose of this 

 paper, it is, I think, useful to point out certain of the more 

 striking of these advances, using Varan osanrus, Deiopeus, and 

 Dimetrodon as the series of forms. These animals lived side by 

 side, and this series is only a morphological one. 



In general shape Dimetrodoii shows an .advance on Yarano- 

 saurus in that the snout is much (.leepened. is square in section 

 onlv immediately in front of the orbit, and is, in general, wedge- 

 shaped with a rounded dorsal edge. The parietal region is 

 narrower, and leaves the tempoial fossfe visible from above. The 

 occiput is more vertical. In the brain-case Dimetrodon shows an 

 advance over J)eiopevs in that the basioccipital is thinner, the 

 hasisphenoidal tubera smaller, and theparoccipital process larger. 

 The anterior margin of the pro-otic lies further in advance of the 

 internal auditory meatus. The brain-cavity is considerably 

 deeper and wider posteriorly. The fenestra ovalis of Dimetrodon 

 is smaller than that of Deiopeus and the stapes lighter. The roof 

 of the skull of Dimetrodon diflers from Varmiosanrns in the 

 following ways : — the parietals are less wide and the pineal 

 foramen fui"ther forward. The postorbital is visible from above. 

 The pair of frontals have acquired a cruciform shape owing to a 

 widening of the interorbital surface. Deiopeus provides an exact 

 intermediate, the increased width of the interorbital surface 

 liaving arisen by an increase in size of the pre- and post-frontals, 

 so as to leave a gap which is filled up by a special process of the 

 frontal . 



In side-view the skull of Dimetrodon shows a shortened and 

 deepened maxilla touching the nasal in a shoi-t suture. The 

 dentition is sharply divided ii\to incisors and cheek-teeth by a 

 diastema. The third maxillary tooth is much larger than the 

 first two, and the lower border of its socket lies well below the 

 dentigerous bordei' of the premaxilla. 



The lachrymal does not reach the septomaxilla. The oibit is 

 placed high up in the skull. 



It seems pi-obable that the deepening of the maxilla and the 

 '•step" depend on the necessity of finding room for the roots of, 

 and the development of, replacing teeth foi- the gieatly lengthened 

 maxillaiy teeth. 



Tjie condition of the canine may depend on the following con- 

 siderations : — A large canine in the upper jaw presupposes a 

 similar tooth in the lower jaw ; such teeth, which are designed 

 for killing animals, are most useful in the front of the mouth. The 

 lower jaw, as a whole, bites inside the upper jaw. The first tooth 

 of the lower jaw cannot be much enlarged, because of the ditficulty 

 of making a pit for its reception near the middle line in the 

 palatal process of the premaxilla ; hence the lower canine cannot 

 be quite at the end of the jaw, A large lower canine almost 

 involves a diastema in the upper jaw for it to bite into, because 



