72 



ME. D. M. S. WATSON ON THE 



being laterally placed, the foi'matioii of a pit in the palate 

 for its I'eception so naiTOws tlie dentigerous part of the maxilla 

 outside it as to leave no room for a tooth. The sfvme factors Avill 

 render difficult any great enlai'gemeiit of the next tooth, and the 

 upper canine thus comes to lie beliind the diastema, and separated 

 from it by at least one small tooth. Considering the mechanics 

 of the whole arrangement, it appears useful to make the dentition 

 more or less symmetrical about the diastema by enlarging a pi'e- 

 maxillary tooth to corresjoond with the canine, so that the single 

 lower canine forces the prey into the gap between two large upper 

 teeth. The dentition so designed is realized in Dimetrodon. 



Text-figure 22. 



A. Superimposed outlines of sagittal sections of the brain-cases of Deiopeits in 



broken line and Dimetrodon in continuous line reduced to the same length, 

 to show the thinning of the basis cranii, the enlargement of the brain-cavitj% 

 and the forward growth of the pro-otic. 



B. Superimposed outlines of sagittal sections of the brain-cases of Leptotra- 



chelus in broken line and Diademodon in continuous line reduced to the 

 same length, to show : the thinning of the basis cranii, the forward growth 

 of the pro-otic, the enormous increase of the cerebellar cavity, and the 

 relatively' slight growth of the cerebral region. 



The great deepening of the maxilla demanded by the canine 

 automatically squeezes out of existence the anterior part of the 

 lachrymal. 



Beiopms and Ophiacodoniprovide nn exact intermediate between 



