288 MAMMALIA. 



was correlated with the incoming of grasses as a dominant 

 feature in the earth's flora. Foremost in the advance the brain 

 has become relatively larger and of a higher type. The brain- 

 case usually exhibits a gradual thickening, often bearing horns 

 and bony prominences, rarely (as in the case of the elephants) 

 expanded for the increase of muscular attachments. Some- 

 times (as in the ruminants) the face becomes sharply bent 

 downwards upon the basicranial axis. The dentition, besides 

 being variously complicated and reduced in extent, exhibits one 

 very constant change ; the molars and premolars in all the 

 earliest forms have very short (brachyodont) crowns fixed on, well- 

 developed roots, whereas the same teeth in the later forms have 

 crowns of gradually increasing height (hypsodont or hypselodont], 

 and eventually these teeth are persistently growing in the young 

 and only become rooted in old age, when they have been con- 

 siderably reduced by wear (see p. 320, fig. 182 E). The neck also 

 undergoes much modification. Usually its flexibility is increased 

 by the slight development of a ball-and-socket joint between 

 the vertebrae and a curving of the zygapophysial facettes. In 

 the elephants it becomes excessively shortened. Except in 

 such persistent types as the elephants and tapirs, the limbs 

 become more and more digitigrade ; the clavicle always de- 

 generates to a ligament, and by the atrophy of the ulna the 

 fore limb loses all power of twisting. At first the five digits 

 are present, while the elements of the two rows of the carpus 

 (to a certain extent also those of the tarsus) are more or less 

 directly opposed, each proximal bone with its own distal bone. 

 Soon this arrangement is strengthened by the interlocking of 

 the two series. Then all the digits become reduced or lost, 

 except a final one or a fused pair. Meanwhile the radius and 

 tibia increase in size at the expense of the ulna and fibula, 

 which become degenerate remnants and usually lose their 

 articulation with the carpus and tarsus ; and the ginglymoid 

 ankle-joint, between the double-grooved end of the tibia and 

 double-ridged astragalus, assumes its maximum perfection. 



