98—64 INDIAN TERTIARY AND POST-TERTIARY VERTEBRATA. 
probability be regarded as a liiglily specialized branch of the Hyotherium stock. 
The structure of the molars of Sanitherium likewise suggests that this genus may be 
another offshoot from the same branch. In some forms of Ilyotheriiim [H. typiis) 
the crowns of the true molars are nearly square, and the premolars are usually 
stouter than those of existing pigs ; and these forms may be regarded as the 'most 
primitive. 
Turning to the genus Siis, it has been shown that the crowns of the molars of 
several of the earlier forms were relatively short, and that the premolars, especially 
those of the lower jaw, were larger in proportion to the true molars than is the case 
among most existing species ; the latter character culminating in the mandible pro- 
visionally referred to S. giganteus. The resemblance of the teeth of that jaw to those 
of Tetraconodon indicates that the group (entelodonts) to which that genus belongs and 
Bus are diverging branches of one stock, which was jDrobably closely connected with 
the primitive hyotheroid stock. In the entelodont branch the premolars appear to 
have gone on increasing in relative size, till they attained their greatest development 
in Tetraconodon itself ; and it is not improbable that some of the anthracotheroids 
may have taken origin from an early branch of the same stock. Tlie extreme 
simplicity of the structure of the molars of Tetraconodon indicates that this genus 
has not descended from Bus. 
In the true pigs, on the other hand, the premolars having attained in the lower 
jaw of one form (? B. giganteus) a comparatively large development, ever afterwards 
began to diminish in relative size, and subsequently in number ; while progressive 
development, both as regards size and complexity of structure, was transferred to 
the true molars, and esj)ecially to the last tooth of that series. Thus while in the 
mandible provisionally referred to B. giganteus m. 1 and m. 2 are remarkably short, 
and the talon of m. 3 is comparatively long, the premolars being of great size ; in 
the form here called B. titan the first two molars have increased in relative length, 
although the last tooth still retains its simple talon ; the lower premolars being still 
relatively larger than in existing pigs. In B. falconeri the early molars have become 
still more elongated and complex, and the last tooth of some individuals has attained 
a length and a complexity of structure unknown among any existing members of 
the genus ; the lower premolars being much narrower than in any of the previous 
forms. This branch appears to have culminated in the existing African genus 
Phacochoeynis^ where the last molar has become extremely narrow and complex, and in 
its tall crown, and in its persistence after all the earlier teeth have disappeared 
foreshadows, or jDarallels, a proboscidian type. In this genus the gradual diminution 
in the size of the preniolars has culminated in the non-development of the first in 
the upper, and of the first and second in the lower jaw, and in the relatively small 
size and early disappearance of the remaining teeth. 
Of the existing species of true pigs, the African river-hogs exhibit traces of a 
primitive type in their relatively large premolars, short m. 1 and m. 2 and small 
