22 



ME. E. LYDEEEEE ON A J AW OF HYOTHEEITJM. 



molars are intermediate between those of Choeropotamus and Sus — 

 those species in which the molars are widest and there is no talon 

 to m. 8 (H. typum) being nearest to the former, while those in 

 which the width of the molars is less and the talon of m- 3 well 

 developed (H. WaUrhousei) come closer to the latter. In certain 

 examples referred by Dr. Filhol (op. tit.) to H. typum there is only- 

 one external column to pm. 4 , which thus shows a retention of a 

 characteristic of Choeropotamus. The latter writer has thought that 

 Hyotherium should not be regarded as the direct ancestor either of 

 Sus or Dicotyles ; but from the character of its dentition, its wide dis- 

 tribution, its geological horizon, and the absence of any other known 

 form which could occupy such a position, I have long had great 

 doubt as to the correctness of this conclusion, and have expressed 

 myself to the effect that the genus must have been at least closely 

 allied to such ancestral form ; and I am now pleased to observe that 

 Dr. Max Schlosser *, of Munich, is of opinion that Hyotherium really 

 occupies a middle position between the modern Sus and Dicotyles and 

 the Upper Eocene (Oligocene) Chceropotamidoe, in the sense in which 

 the latter term is employed by Prof. Flower and myself f. Starting 

 from that family, a line of evolution may be traced in one direction 

 from the type genus to Anthracotherium, Hyopotamus, and the 

 tetracuspid Selenodonts, while another line may be traced through 

 Cehochoerus to Hyotherium, Hippohyus, Dicotyles, Sus, and Phaco- 

 choerus. Dicotyles has attained an excessive specialization in respect 

 of the upper premolars (which are as complex as the true molars), 

 while in Sus the specialization has been more confined to the true 

 molars ; and it is noteworthy that in the specialization of the cheek- 

 dentition of the higher species of the latter genus, while the premolars 

 and first and second true molars only gain a moderate increase in 

 height and complexity, the last true molar becomes enormously 

 developed posteriorly, and does not come into use until the first true 

 molar is almost worn away. This line of specialization culminates 

 in Phacochcerus, where all the anterior cheek- teeth may disappear in 

 the adult, and to a certain extent is analogous to the peculiar 

 dental development characteristic of the Proboscidea. Those species 

 of Sus which present the greatest specialization in this respect 

 occur in the later Tertiaries of India and North Africa, the common 

 living species (S. cristatus) of the former country being probably a 

 descendant of the group which has lost the extreme dental develop- 

 ment characteristic of the Pliocene species (S. Falconeri) As 

 examples of species retaining a primitive type of dentition, may be 

 mentioned Sus andamanensis of the Andamans, Sus barbatus of 

 Borneo, and the Kiver-hogs (Potamochcerus) of Africa. At least in 

 the case of the first-mentioned species the retention of a generalized 

 character may be attributed to the absence of competition. 



* Morphol. Jahrbuch, vol, xii. pp. 89-92 (1886). 



t Dr. Max Scblosser prefers to place Cehochoerus in the Suidas, and to merge 

 Choeropotamus in the Anthracotberiidfe. 



X For the relations of these species see ' Palasontologica Indica,' ser. 10, 

 vol. iv. pt. 2 (1886). 



