RODENTS.
steel laid upon the back of a chisel; whence the name, ‘ dentes
scalprarii,’ given to the incisors of the Rodentia.(1)
The varieties to which these incisors are subject in the different
Rodents are limited to their proportional size, and to the colour
and sculpturing of the anterior surface. Thus in the Guinea-pig,
Jerboa, and Squirrel, the breadth of the incisors is not half so
great as that of the molars, whilst in the Coypu they are as broad,
and in the Cape Mole-rats, (Bathyergus and Orycteromys), broader
than the molars.
In the Coypu, Beaver, Agouti, and some other Rodents, the
enamelled surface of the incisors is of a bright orange or reddish
brown colour. In some genera of Rodents as Orycteromys, Otomys,
Meriones, Gerbilla, Hydrochcerus, Lepus and Lagomys, the anterior
surface of the upper incisors is indented by a deep longitudinal
groove. This character seems not to influence the food or habits
of the species; it is often present in one genus and absent in
another of the same natural family: in most Rodents the anterior
enamelled surface of the scalpriform teeth is smooth and uniform.
The molar teeth are always few in number, obliquely implanted
and obliquely abraded, the lateral series converging anteriorly in
both jaws; but they present a striking contrast to the incisors
in the range of their varieties, which are so numerous that they typify
almost all the modifications of form and structure which are met
with in the molar teeth of the omnivorous and herbivorous genera
of other orders of Mammalia.
In some Rodents the molar teeth are rootless, like those of the
Wombat, the Toxodon and Elasmothere; some have short roots
tardily developed, like the molars of the Horse and Elephant; and
some soon acquire roots of the ordinary proportional length. (2)
(1) John Hunter grouped together the quadrupeds composing this order under the name
of ' Scalpris-dentatabut the large curved, chisel-shaped incisors, though common to all the
Rodents, are not peculiar to that order: we have seen them in the Wombat amongst the
Marsupials ; they are present in the Cheiromys amongst the Lemurine group of Quadrumana,
and in the Toxodon amongst the Pachyderms; some of the Shrews have the anterior incisors
restricted in number and developed in bulk, almost to their proportions in the Rodentia.
(2) Prof. Erdl has given a tabular arrangement of the molars of the Rodentia, according
RODENTS. 401
The Rodents which have rootless molars, comprise the families
of the Hares,(1) Chinchillas,(2) Chili-rats,(3) and Cavies(4), most of
the Voles,(5) the Houtias (Capromys), and the Cape Jerboa (Helamys).
The genera which have molars with short or incomplete roots,
developed late, are Castor (Beaver), Hystrix (Porcupine),(6) Coslogenys
(Spotted Cavy), Dasyprocta (Agouti), Spalax (Blind-rat), Myopota-
mus (Coypu), Euryotis, Ascomys, and Aplodontia.
The families of the Squirrels, Dormice, Rats, and Jerboas have
rooted molars.
The differences in the mode of implantation of the molar teeth
relate to differences of diet. The Rodents which subsist on mixed
food and which betray a tendency to carnivorous habits, as the true
Rats, or which subsist on the softer and more nutritious vegetable
substances, as the oily kernels of nuts, suffer less rapid abrasion
of the molar teeth; a minor depth of the crown is therefore needed
to perform the office of mastication during the brief period of
existence allotted to these active little Mammals; and, as the economy
of nature is manifested in the smallest particulars as well
as in her grandest operations, no more dental substance is developed
after the crown is formed, than is requisite for the firm
implantation of the tooth in the jaw.
Rodents that exclusively subsist on vegetable substances, especially
the coarser and less nutritious kinds, as herbage, foliage, the
bark and wood of trees, wear away more rapidly the grinding surface
of the molar teeth; the crowns are therefore larger, and their growth
continues by a reproduction of the formative matrix at their base
in proportion as its calcified constituents, forming the exposed
working part of the tooth, are worn away. So long as this reproductive
force is active the molar tooth is implanted, like the incisor,
to their modes of implantation, in his excellent Paper on the Teeth of that order, in the Munich
Transactions, * Abhandlungen der K. Bayerischen Akademie der Wissenschaften/ Bd. in.
1841, p. 529.
(1) Lepus, Lagomys,
(2) Lagostomus, Lagotis, Chinchilla.
(3) Abrocoma, Octodon, Schizodon, Poephagomys, Ctenomys.
(4) Hydrochcerus, Dolichotis, (PI. 104, figs. 2 & 3), Kerodon, Cavia.
(5) Lemmus, Arvicola, except A. riparia.
(6) PI. 104, fig. p
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