O. C. Marsh— Vertebrate Life in America. 377 
The real progress of mammalian life in America, from the 
beginning of the Tertiary to the present, is well illustrated by 
the Brain-growth, in which we have the key to many other 
ceans with teeth retain this type, except the Zeuglodonts, which 
approach the dentition of aquatic Carnivores. In the higher 
mammals, the incisors and canines retain the conical shape, and 
the premolars have only in part been transformed. e latter 
gradually change to the more complicated molar pattern, and 
hence are not reduced molars, but transition forms 1 
the cone to more complex types. Most of the early Tertiary 
mammals had forty-four teeth, and in the oldest forms the 
premolars were all unlike the molars; while the crowns were 
short, covered with enamel, and without cement. Each stage 
of progress in the differentiation of the animal was, as a rule, 
_tnarked by a change in the teeth; one of the most common 
being the transfer, in form at least, of a premolar to the molar 
series, and a gradual lengthening of the crown. Hence, it is 
often easy to decide from a fragment of a jaw, to what horizon 
of the Tertiary it belongs. The fossil Horses of this period, 
for example, gained a grinding tooth, for each toe they lost, 
one in each epoch. In the single-toed existing horses, all the 
premolars are like the molars, and the process is at an end. 
Other dental transformations are of equal interest, but this 
illustration must suffice. 
The changes in the limbs and feet of mammals during the 
Same period were quite as marked. he foot of : the primitive 
mammal was doubtless plantigrade, and certainly five-toe 
