1 902.] Osborn, Dolichocephaly and Brachycephaly in Mammals. 8 1 



awaits confirmation. The skulls of these Titanothere species 

 differ from those of man and of Rhinoceroses in the fact that 

 the cranium, or skull proper, does not vary in width so widely 

 as the cheek arches or zygomata. It is chiefly the enormous 

 expansion of the latter bones in Brontotherium elatum which 

 makes the skull actually as broad as it is long. But while 

 mainly a zygomatic expansion, that there is a very pro- 

 nounced cranial and facial expansion is attested by the broad 

 palate, relatively short and crowded dental series, trans- 

 versely expanded horns, abbreviated nasals, short malar 

 bridge in front of the orbit, abbreviated mastoid and paroc- 

 cipital portion behind the external auditory meatus, trans- 

 versely expanded occiput and occipital condyles, broad 

 exoccipital and postglenoid processes, short, deep, and thick 

 lower jaw with less prominent angle. There is no mistaking 

 a typical brachy cephalic for a dolichocephalic jaw, every 

 contour and proportion is different. Analogous differences 

 are observed among the Rhinoceroses. 



The above are only a few of the correlated effects of skull 

 proportion. In the comparison of all the Titanotheres from 

 the beginning to the end of their remarkable history it is 

 found that the primitive and central types are mesaticephalic, 

 and the divergence is into brachycephaly and dolichocephaly. 

 The following table presents the extremes of structure as 

 observed especially in the Titanotheres. 



Correlated Skull Characters. 



BRACHYCEPHALY. DOLICHOCEPHALY. 



Teeth. 



Dental series Crowded Elongate. 



Diastemata Closed Increased. 



. . , , „ , ( Persistent and 



Anterior premolars, J Suppressed. j _, 



P"' \ 1 O^^ f^^^ suppressed. ) TwoTangs retained. 



Intermediate tuber- 

 cles of molars Persistent Reduced. 



Opposite dental ser- 

 ies Convergent or arched More parallel. 



[yanuary, igo2.'\ 6 



