88 Bulletin American Miiscnvi of Natural History. [Vol. XVI, 



of the metapodials. So complete is the correlation that we 

 can, for example, immediately distinguish between the ele- 

 ments of the pes of a long-skulled and of a short-skulled 

 Rhinoceros. 



Eliminating all the exceptions, there appear to have been 

 two general causes for the elongation of the skull. First, the 

 elongation of the face for the accommodation of very long 

 hypsodont grinding teeth in front of the orbit, as observed 

 by Kowalevsky. Second, the elongation of the skull as a 

 whole, correlated with the elongation of the limbs, an adap- 

 tation to grazing and cursorial habit. 



No Adequate Theory of Brachycephaly. 



It is much more difficult to account for progressive brachy- 

 cephaly. An adequate theory of its causes is still wanting, 

 as shown by the following examples : 



Among Primates the shortening of the skull takes place 

 pari passu with the increasing use of the manus in conveying 

 food to the mouth; this is well illustrated by the contrast 

 between the quadrupedal, long-skulled baboons and the more 

 bi-pedal short-skulled monkeys. 



We are especially at a loss to offer any adequate explana- 

 tion of the causes of progressive brachycephaly in mammals 

 which seem to suffer thereby a reduction and compression of 

 the dental series. In certain Titanotheres and Rhinoceroses 

 the shortening of the skull seems to crowd and diminish the 

 usefulness of the teeth, an apparently inadaptive process. 



The observations of Nathusius led him to the conclusion 

 that among the Suidae abundant food tended to shorten and 

 broaden the head and the face. Darwin observes that 

 domestication tends to shorten the bones of the face in many 

 animals. 



Among Carnivores, and among the long-horned Titan- 

 otheres, abbreviation of the skull favors the effective -use of 

 the canine tusks and of the paired horns respectively. But 

 brachycephaly also develops to an extreme in certain defence- 

 less types, such as Cyclopidius among the Oreodonts. 



