No. 3.] MODE OF EVOLUTION IN THE MAMMALIA. 375 



larged, but lightened by large sinuses or air-cells. The ribs are 

 very long, and strongly arched, to carry the great mass of 

 viscera. The scapula is triangular in shape, and in the Dino- 

 cerata extremely like that of the elephant, with greatly elon- 

 gated suprascapular border and very large post-scapular fossa, 

 prominent and massive acromion. The humerus also is much 

 alike in these two groups, but that of Titanotherinm is more 

 rhinoceros-like. In all, the ulna is unreduced or even enlarged, 

 and the carpus is composed of massive cubical blocks. The 

 ilium is in all greatly everted and immensely broadened in the 

 transverse direction. The femur is long and massive ; that of 

 UintatJierium is almost an exact copy of that of the elephant, 

 having neither third trochanter nor pit for the round ligament, 

 both of which are preserved in the earlier members of the Am- 

 blypoda, e.g. Coryphodoji. In all, the tibia is short, and the fibula 

 complete ; the astragalus extends far over on the cuboid, though 

 separated from it in the elephant by the navicular, and is little 

 or not at all grooved, so that the maximum of motion is at the 

 knee-joint. The tarsus of UintatJierium is in general character 

 very much like that'of the elephant, while that of Titanot/zerium 

 does not lose its perissodactyl structure. 



The metapodials are short and massive, especially so in the 

 Dinocerata and Proboscidea, and the ungual phalanges are 

 reduced and nodular in shape, showing that in all the foot was 

 enveloped in a pad. It seems probable that great size and 

 weight of body stops in most cases the process of digital reduc- 

 tion. In Titanotherium the number is, it is true, only IV. in 

 the manus and III. in the pes ; but then this degree of reduc- 

 tion is already reached in the Bridger genus, Palceosyops, which 

 is of moderate size. The other two series are pentadactyl, 1 and 

 in all the foot is nearly isodactyl ; i.e. the digits are all of 

 approximately the same length and thickness, though in Titano- 

 therium there is more inequality than in the others. In all 

 three groups there are no medullary cavities in the long bones, 

 the interior of which is filled with cancellous tissue. 



1 To this statement one exception must be made. The specimens of naviculars of 

 Dinothcriian giganteum, from Pikermi, preserved in the Paris museum, show only a 

 very small facet for the first cuneiform, while in the D. Bavaricum of the Vienna 

 museum there is no facet for that bone. This points to a great reduction or loss of 

 the hallux. The same statement will apply to a rather small, Uintatherium-\\kQ 

 navicular in the Princeton museum. 



