342 
ARTIODACTYLA. 
have the intermediate condition Tlie end of the series is seen in 
Auchenia, ^here tlie formula is 
The evolution of the existing types of Camelidcz is a good illustration 
of the operation of the laws of acceleration and retardation. In evidence 
of this, we may follow the growth of the foot and dentition of the most 
specialized, and therefore the terminal genus of the series, the American 
Auchenia. It is well known that the animals of this genus, in common with 
other Ruminants, have the constituent metapodials of the cannon-bone 
distinct during a longer or shorter portion of foetal life. As these elements 
are permanently distinct in the oldest or Miocene genus PoehrotJierium, it is 
evident that acceleration of the process of ossification has caused their 
union at successively early periods in the genera of later ages. This is 
indicated by the long duration of their separation in the Loup Fork genus 
Procamelus. It is also well known, since the time of Goodsir, that the 
embryos of Ruminants exhibit a series of superior incisor teeth, which 
disappear early. It is probable, but not certain, that, in the Miocene genus 
PoebrotJierium as in various contemporary Selenodont Artiodactyla, the 
superior incisors persisted. I have, however, discovered that these teeth 
persisted in the Loup Fork genus Protolabis during adult life. I have also 
found that one (the second) of these teeth in Procamelus occidentalis persisted 
without being protruded from the alveolus until nearly adult age. In genera 
(e. ^., the Bunodont Artiodactyla) where the incisors are normally developed, 
they appear at about the same time with the other teeth, and continue to 
develop to functional completeness. This development is retarded in Pro- 
tolahis, since they are not so matured as to remain fixed throughout life in 
their alveoli. In Procamelus, the retardation is still greater, since the first 
incisor reaches very small dimensions, and is with its alveolus early removed, 
while the second incisor only grows large enough and for a sufficient time 
to occupy a shallow alveolus, without extending beyond it. In the first 
incisor, the process of retardation has reached its necessary termination, 
i. e., atrophy* or extinction, while in the existing Camelidce the second 
incisor also has disappeared in the same way. In Ruminants other than 
Camelidce, the third or external incisor has undergone the same process, 
* See Pi’oc. Acad. Pliila., 1870, p. 17, for an explanation of these terms. 
