Animal Dentition 7) 
and in consequence of this the jaws must have had a vertical champing action instead 
of a horizontal grinding movement. Again, in most mastodons the upper tusks are 
bent downwards, instead of curling upwards lke those of the elephants, and some 
species of the former were also furnished with a pair of tusks in the lower jaw. 
From such four-tusked mastodons there is a transition to other extinct species in which 
the tusks are but slightly larger than the other incisors (which are retained), while the 
molar teeth depart but little from a normal type. 
Altogether unique is the type of dentition presented by the rodents, or gnawing 
mammals, such as squirrels, beavers, porcupines, hares, etc. With the exception of the 
hares, rabbits, and picas, in which there is an additional small, functionless pair behind 
those of the upper jaw, the front teeth in both jaws are reduced to a single pair of large 
ever-growing incisors, faced with enamel only on thew broad front surfaces, and kept 
worn to a sharp chisel-like edge by constant use. As these teeth grow im the arc of a 
circle and are only kept of the proper length by wear, it is obvious that if one be broken, 
its fellow of the opposite jaw will, as in the above-mentioned case of the wild boar, 
continue to grow till its point re-enters the jaw close to the root. Behind these chisel- 
like incisors comes a long gap, without a vestige of a tusk, and then follow the molars, 
usually three or four in number in each jaw. In many rodents, such as squirrels and 
mice, the molars are low-crowned and capped with low tubercles, so as to be very hke 
miniature mastodon-teeth. Other groups, however, such as the voles, have the crowns of 
their molars formed of closely-packed vertical plates, sumulating the corresponding teeth of 
the elephants. This type of structure attains its maximum development in the large South 
American rodent known as the carpincho, or capibara, in which the last molar (as shown 
in Fig. 17) is much longer than the rest, and is composed of twenty or more plates. 
Everyone who has watched a rabbit or squirrel feeding will have noticed that the jaws 
of these animals work with a backwards and forwards, instead of a sideways, movement, 
this action being characteristic of all rodents; and in order that it may be able to take 
place, the lower jaw is articulated to the skull in an altogether pecular manner. 
The long gap between the incisors and the molars is doubtless connected with this 
action, as it also is with the habit of storing food in cheek-pouches characteristic 
of many rodents. Absolutely unique as is the dentition of modern rodents, there 
are nevertheless indications among fossil forms of a transition towards a normal type. 
Since the teeth of the bats and their near relatives the terrestrial insect-eating 
mammals, such as hedgehogs, moles, shrews, etc., are all formed on a small scale, their 
description and illustration would be difficult in an article of the present nature. It 
may be observed, however, that while the fruit-bats or “ flying-foxes” have smooth and 
low-crowned teeth, traversed by a median longitudinal groove, admirably adapted 
for mashing-up fruits like bananas, in 
the insectivorous species the crowns of 
the cheek-teeth are studded with sharp 
cusps, equally well suited to pierce the 
hard integuments of beetles and other 
insects. Many of the Insectivora have 
also teeth of the same type. On the 
other hand, the curious flying lemur of 
the Malay countries, which is generally 
meluded in the same group, has 
the lower front teeth with a remark- 
able comb-like structure of a type 
unknown elsewhere in the whole animal 
kingdom. 
Tig. 16. Side view of the Dentition of a Mastodon. 
