1873.] 



Osteology of the Hyopotamidse. 



153 



os niagnuin ; the second digit of the pes is supported by the second cunei- 

 form, and by its fibular projection is connected with the third cuneiform. 

 The first digit is lost in all Ungulata, and its typical bone, the trapezium, 

 or first cuneiform, helps to support the second digit. 



Diagram of a Typical Foot in Ungulata Paricligitata. 

 Manus. Pes. 



Unciform. 



IV. 



Os 



Trape- 



Trape- 



magnA 



zoid. 



/ zium. 



III. 



Cuboid. 



IV. 



Cunei- 

 form 3. 



III. 



Cunei- 

 form 2. 



Cunei- 

 form 1. 



Beginning from this typical structure of the manus and pes, which was 

 probably exhibited by the progenitors of the Paridigitata, we may fol- 

 low its gradual reduction along both lines of descent in the crescent- 

 toothed (Selenodont) and tubercular-toothed (Bunodont) Paridigitata. 

 Both lines present a series of parallel modifications, and the parallelism is 

 often carried to the minutest details. The only difference is, that along 

 the crescent-toothed line {Selenodonta) the reduction is proceeding at a 

 much quicker rate than along the tubercular-toothed {Bunodonta). The rea- 

 son of this may consist perhaps in the commencing faculty of rumination 

 in the former group, which faculty gave it an immense advantage over the 

 latter. For the comparative anatomist this slow rate of change in the 

 Suina is exceedingly welcome, as it brings the modification of the Suil- 

 line foot to our own time, and allows us to discover all the intermediate 

 stages of modification, which, being passed over very rapidly, and in 

 ancient periods, by the crescent-toothed group, have left none or but few 

 traces of their existence. 



By the reduction of the foot in Paridigitata, I simply mean that the 

 function of locomotion which has been performed primitively by all the 

 four (or five) digits begins to be carried on chiefly by the middle two, the 

 lateral digits undergoing a gradual decrease. This, as I have said before, 

 seems to be of great advantage to the organism, and is manifested by 

 all descending lines of Ungulata. 



In trying to ascertain the exact method of this reduction and its final 

 results in recent and fossil genera, we come to very interesting facts that 

 have not been duly noticed before, and which furnish us with the expla- 

 nation of the presence of so many very reduced forms even in old Eocene 

 and Miocene deposits. In both groups, the crescent-toothed {Selenodonta) 

 and the tubercular-toothed Paridigitata {Bunodonta), we meet with a two- 



