560 | U. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
FIBER, Cuvier. 
Fiber, Cuvier, ‘‘Lecons d’ Anat. I, 1800.” 
Form somewhat arvicoline, except the tail, which islong, much compressed vertically, with scant hair; hind feet partly 
webbed ; fore feet with stiff bristles. Teeth arvicoline, the posterior lower molar composed of four or, perhaps, five prisms, 
the re-entrant angles alternating, (if four, the anterior with a re-entering angle.) 
The genus Fiber, based on a single species—the well known muskrat—is confined to North 
America, where it has an extensive range, except, perhaps, in its extreme northern and south- 
ern extremities. Although, in many points similar to the Arvicolas, yet the peculiarities of 
the tail and feet are not found in any Arvicola, all of which it greatly exceeds in size. The 
skull, too, although very similar in the two genera, yet possesses some appreciable distinctive 
characteristics. 
The outlines of any single series of molars are formed by two straight lines approximating 
a little behind, more so above than below. The first upper molar is longest; the third is longer 
than the second, though not so wide. The crown of the first has two re-entrant angles on 
each side (and, of course, three salient ones); the second, two outside and one inside; the 
third, three outside and two inside. There are five prisms on the first molar, four on the 
second, and five on the third ; these are mostly sub-triangular. The enamel loops, in all cases, 
alternate with each other, and extend across to the opposite enamel, completely isolating the 
dentine in each prism. In the first molar, the anterior indentation or re-entrant angle is 
internal, in the other two it is external. 
In the lower jaw the first molar is much larger than the others; the second and third about 
equal in length. The first has five indentations or re-entrant angles inside and four outside ; 
the second and third have each two on each side. These, with the triangles separated by 
them, are alternate, not opposite; most anterior in the first molar inside; in the others outside, 
as in the upper jaw ; when the corresponding teeth of the two jaws are superimposed, however, 
these conditions are reversed. There are nine prisms in the first molar, and five in each of 
the others. 
The re-entrant enamel folds in all the teeth extend across to the enamel on the opposite side, 
except in the case of the anterior pair of the first lower molar, which do not meet, and, perhaps, 
the posterior pair of the last upper molar. 
The upper incisors are rather broadest; plane anteriorly, but considerably bevelled or 
rounded off at the outer edge; the lower are still more bevelled externally, so that their section 
is a spherical triangle with rounded corners. 
A striking peculiarity of the skull in Fiber consists in the great expansion over the head, of 
the temporal bone, which, by the great compression between the orbits (the skull reduced to a 
less width than that of the end of the muzzle,) anteriorly forms a right angle projecting far into 
or over the orbit, the anterior legs of the two angles on opposite sides of the head in a straight 
line perpendicular to the axis of the head. The parietals are reduced to a small sub-orbicular 
space with various angles, The interparietal bone is nearly as long as broad, and one-third as 
large as either parietal. The occipital foramen is as broad as high. 
