ARTIODAOTYLA. 
341 
The latter tooth is wanting from the lower jaw of the P. ImmpJiresiana. 
The difference in dental formula between the superior and inferior dental 
series admitted provisionally in Pliauclienia finds justification in the formula 
of the Llamas {Aiiclienia)^ where the premolars are p. 
The line of descent of the Horses has been already traced by several 
paleontologists. Another series has been nearly completed by recent 
discoveries,* viz, that of the Camels. I have already indicated f the ante¬ 
cedent relation in which the Miocene genus Poehrot]ierm7n stands to the 
existing Camels in the structure of the limbs and teeth, as well as the 
intermediate position occupied by Procmielus in the characters of the incisor 
teeth. It now remains to point out the relations determined by the structure 
of the feet in Proca^nelus, and the dentition in Protolahis, and as described in 
the preceding pages. Commencing with the earliest genus, Poehrotheriunt, we 
have the molar teeth 4-3, as in the primitive Mammalia generally. There 
are but two elongate metacarpals, which are not united into a cannon-bone; 
the lateral ones being rudimental, while the carpals are of the number 
characteristic of the Ma^nmalia of all the orders with numerous toes, 
namely, seven. In Protolahis of the succeeding formation, the molar 
formula continues to be 4-3, but the posterior teeth are more prismatic in 
form than in the Miocene genus. The incisor teeth are present, thus dis¬ 
playing the primitive character of the class generally; though, as these 
teeth are easily shed, an approximation to the edentulous condition existing 
in this part of the mouth of Ruminants is apparent. It is in Procamelus that 
the incisor dentition of the existing Camelidce is first seen, but that genus 
still retains the molar dentition of the primitive character, the formula being 
Pm. |; M. I. In the feet, the approximation to the existing Camelidce is 
greater than in the dentition. Thus, the lateral rudimental metacarpals of 
Poebrotlie7%wn have disappeared, and with them the trapezoides of the carpus. 
The magnum remains distinct, while the middle metacarpals are united at 
full age into a cannon-bone. In the contemporary genus PliaucJmzia, a 
further modification of dentition is observed. As above stated, the molars 
of Procamelus number in Caznelus, they number in Pliauclienia, we 
* See Proc. Acad. Phila., 1875, p. 261. 
tBull. U. S. Geol. Siirvs. Terrs., No. i, 1874, p. 25; Ann. Etpt. 1873 (1874), p. 500. 
