210 NORTHERN ZOOLOGY. 
Aptopontia. (Richardson.) Sewellel. 
Aplodontia. RicHarpson, Zool. Journ. January, 1819. 
CHARACTERS. 
Dental formula ; incisors $, canines 3, grinders = = 22, 
Incisors, very strong, flatly-convex anteriorly without grooves; narrower behind. Grinders 
simple, remarkably even on the crowns. The first in the upper jaw, small, cylindrical, and 
pointed, is placed within the anterior corner of the second one, and exists in the adult. The 
rest of the grinders are perfectly simple in their structure, without roots, and have slightly 
concave crowns, which are merely bordered with enamel without any transverse ridges or 
eminences. On the exterior side of the four posterior pairs of upper grinders, and the inner 
side of all the lower ones, there is an acute vertical ridge extending the whole length of the 
tooth, formed by asharp fold of enamel. When the grinders are in situ, there is a wide 
semicircular furrow between each pair of ridges, formed by the two adjoining teeth. The side of 
each tooth opposite the ridge is convexly semicircular. ‘The second grinder in the upper jaw, 
and the first in the lower one, are a little larger than the more posterior ones, and the former 
has a projection of enamel at its anterior corner, producing a second though smaller vertical 
ridge, within which the first small grinder is situated and leans towards it. There is a slight 
furrow on the exterior sides of the lower grinders, most conspicuous in the first one. 
Palate narrow, bounded by perfectly parallel and straight rows of grinders. _ 
Head flat and broad, nose a little arched, thick and obtuse. Lower jaw thick and strong, 
with a large triangular process, concave behind, projecting at its posterior inferior angle further 
out than the zygomatic arch. The transverse diameter of the articulating surface of the 
condyle is greater than the longitudinal one. The jaw is altogether stronger than is usual in 
the Rodentia. 
'  “ Cheek-pouches none” *, 
Eyes very small. Ears short and rounded, approaching in form to the human ear, and 
thickly clothed on both sides with short hair. 
Body thick and short, clothed.with fur like that of a musk-rat, but not so long or fine. 
Limbs robust, short ; feet moderately strong, with naked soles. Five toes on all the feet, 
rather short, but well separated. The thumb of the fore-feet is considerably shorter than 
the other toes. Claws, particularly the fore-ones, very long, strong, much compressed, and 
but little curved. . 
Tuil very short, concealed by the fur of the hips. Mamme six, the anterior pair situated 
between the fore-legs. 
Habits —Animals forming rl societies, feeding on vegetable substances, and living in 
burrows, 
* Mr. David Douglas, 
