584 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
cartilaginous,—the only cartilage that entirely surrounds the wind-pipe. It is 
infundibuliform, the diameter of the ring being considerably greater above 
than below; and it is not so high in front as behind, where it bears the ary- 
tenoids. These are well developed, entirely cartilaginous, and of an irregular 
shape, impossible to describe concisely; when in mutual SEE SIUS, they 
reach nearly half way across the top of the cricoid ring. 
The trachea is flattened, especially behind, and diminishes somewhat in 
calibre as it passes down, ending in the bronchia with a simple bifurcation. 
None of the tracheal rings are cartilaginous posteriorly, the membranous por- 
tion being nearly the semi-circumference. These half-rings are about thirty 
in number; of the similar bronchial half-rings, there are five or six to the 
first bifurcation. The tracheal cartilages are not regular, either in position, 
size, or Shape; some are not directly transverse to the axis of the tube; some 
are thicker than others; and some are partially divided into two on this or 
that side. The tube is about two inches long. The left bronchus is rather 
longer than the right. 
The /ungs appear to be very small in comparison with the size of the ani- - 
mal. In the collapsed state observed, and with the lobes placed as nearly as 
possible in the natural position, the left lung was less than one and a half inches 
long; the right was a little more. The two differ remarkably in their lobation 
(at least in the specimen examined). _'The left lung consists of only two lobes, 
the lower much larger than the upper; they are almost completely separated, 
only connected by a thin band of parenchymatous tissue; each receives a fork 
of the bronchus. The right lung is much more complicated, being divided 
into four very distinct lobes, each of which receives its“own branch of the 
bronchus; as in the case of the left lung, the parenchymatous connection of. 
the lobes is slight. The third lobe counting from above is the principal one, 
representing more than a moiety of the lung. Above and to the front of this 
main lobe, overlying it much as the auricles of a heart rest upon the ventricles, 
are two small narrow and thin lobes, side by side, of about equal size and 
similar shape. The fourth lobe lies on the inner back side of the main one, 
and equals it in length; but itis very thin and “ straggling”, having about the 
same capacity as one of the small upper lobes. This lobe has but the slightest 
parenchymatous connection with the main lobe, and is furthermore itself 
lobulated. 
Digestive organs —Within the apparent edges of the lips short fine 
