Ixxs Rodentia. MAMMALIA. Rodentia. 
Report, 1877 — funiislies a vast amount of valuable 
matter on this subject, anti we deem it fortunate that 
we are enabled to present in this work a suitable 
digest of its contents under the appropriate heads. 
This order is plainly distinguished by the peculiar 
shape of the cutting-teetli, which are singularly uniform 
in appearance throughout the whole group. It is char- 
acterized b}'- having two incisors in each jaw, with one 
exception — the Leporidce — which have a second rudi- 
mentary jmir behind the upper incisors. 'I’lie cutting- 
teeth are very large and long, and grow continuously 
in the form of an arc, or segment of a circle, which, 
were they not regularly worn at the points by mas- 
tication, would so develop as to form a curved tusk, 
or penetrate the roof of the mouth ; in either event 
the creature is deprived of the usual means of sub- 
sistence, and dies. This happens occasionally, through 
accident, in several groups; the Woodchuck, Arc- 
tomys, and Hats are known to present remarkable 
instances. The wise provision here of a hard 
enameled exterior, and a softer inferior surface, se- 
cures a keen chisel-shaped edge, which serves for 
gnawing, a habit that suggests a name for the order 
— Rodentia, from the Latin, rodere, to gnaw. At 
the roots of the cutting-teeth are pulps, in which 
are constantly forming new substances for their 
growth. The continuous wear of a surface so un- 
equal results in securing a cutting instrument, which 
has been imitated by man. Axes are made in the 
same manner, by introducing a piece of steel between 
two softer plates of iron, the wear of which produces 
similar results. In some instances the molars have 
the same provision as the incisors, the pulp furnish- 
ing a continuous growth. In extreme age some of 
the molars, from a lack of supply from the persistent 
pulp, throw out slender fangs that answer the jmr- 
pose of roots, and then the upward growth ceases. 
'I’he molars of llodents generally are very compli- 
cated in structure, consisting of three portions — 
the hard enamel, which covers the dentine, or soft 
bony portion, and the cement, or crusta petrosa, 
which encircles the exterior of the enamel. Between 
the incisor teeth and the molars there is a wide 
space, the canines being entirely absent. The con- 
dyle of the lower jaw is rounded, and slides along 
the glenoid cavity longitudinally. The tibia and 
fibula of most Bodents are separate; in Muridee 
and Leporidce they are united, or anchylosed below. 
All have the clavicles more or less developed ; the 
Porcupines have none, and those of the Hares are 
quite reduced in size. The feet have the normal 
number of five toes, the thumb being more or less 
I'udimentary. The hind feet have five toes in most 
genera, but in some only four are seen ; there are 
only three in Cavia, Uijius and Dasyj)rocta. All 
have prehensile feet, excepting the Hares and Cavies. 
The intestinal canal is long in most, and has a well- 
formed coecum ; in Myoxina there is none. 
■Members of this order are known to all portions of 
the globe, and are particularly abundant in America, 
where there are said to be as many as exist in all 
other parts of the world together. The order con- 
tains more species than any other of the class of 
mammals, between eight hundred and nine hundred 
being known ; and they are very evenly distributed. 
In South America there are more than in the 
northern half of the New World, but the great num- 
bers of species included in the one genus, Hesper- 
omys, cause this preponderance. There are no 
indigenous Rodents common to both Europe and 
America, excepting the Beaver, which is regarded by 
some as the same in both countries. The Jlusk- 
rat, and a species of Spermophilus are said to occur 
in Kamtschatka; in this event these would hold as 
exceptions. There are no South American species 
in North America, but a close relationship exists be- 
tween the genera. Wallace gives the following as 
regards the families of Rodents, and number of 
genera and species: Muridee has thirty-seven genera 
and three hundred and thirty species. The true 
Mouse (Mils'), he records, is not found indigenous in 
North nor South America, nor in three insular 
groups belonging to the Australian region, but is 
indigenous in the remaining portions of the globe 
very widely. Hesperoinys represents it in the New 
I\''orld ; eighty or more species are enumerated. 
Spalacidae is exclusively of the Old World, and has 
seven genera and seventeen species. Dipodidm has 
three genera and twenty-two species. This is an 
Old World group, excepting Jaculus, or Meriones, 
with one species. Myoxidee has one genus and 
twelve species, all foreign. Saccomyidoe has six 
genera and thirty-three species, all North American. 
Gastoridee has one genus and two species, Ajiierican 
and European. Sciuridee has eight genera, and one 
hundred and eighty to two hundred species. These 
are largely from North America and Europe and 
Asia, but are absent in West Indies, Australia and 
South America, as far north as Paraguay. Hap- 
liodontidcB has one genus and two species, both 
American. Ghincliillidce has three genera and si.x 
species, all South American. Octodontidm has eight 
genera and nineteen species. South American and 
African. Eehimydce has ten genera and thirty 
species, which are found in South America and 
Africa. Gercolaiidce has three genera and thirteen 
to fifteen species, all American. Hystricidae has 
three genera and twelve species, all Old World. 
Gavidee has six genera and twenty-eight species, all 
American. Lagomyidee has one genus at\d eleven 
species, common to both worlds. Leporidce has one 
genus and from thirty-five to forty species, common 
to both hemispheres. 
The two sub-orders are regarded as very clearly de- 
fined. In the first sub-order, Simplieidentati, the 
incisors, or cutting-teeth, are limited to two in each 
jaw, the enamel being entirely confined to the front 
surface of each tooth. 'J'here is a true alisphenoid 
canal, and an external one in the skull. The fibula 
never articulates with the calcaneum. 'J'he testes are 
abdominal, but descend periodically. Nineteen fam- 
ilies are enumerated as belonging to this sub-order. 
The Duplicidentati are characterized by having four 
cutting-teeth in the upper jaw and two in the lower; 
the external ones in the upper jaw are very small, and 
situated somewhat subordinate to the others. In early 
