MAMMALIA, 545 
anterior nares at the front end of the facial region. The 
nasal chambers serve the dual functions of smell and of 
respiration. The length of the nasal chambers and the 
distance between the anterior and posterior nares, combined 
with the great exposed surface of the turbinals, ensure the 
activity of the olfactory sense. 
In the Ce¢acea the erial olfactory sense is of little or no 
use, whilst a rapid and easy passage of air to the lungs is 
essential. Hence the anterior nares have progressed back- 
wards till they come to lie vertically over the internal nares, 
and the nasal “chamber” of terrestrial types, with its 
complex turbinals, has been converted into a simple pair of 
short passages, with no turbinals, leading directly downwards 
to the glottis. In terrestrial types the roof of the nasal 
chamber is formed by the nasals and partly the frontals. 
Here the nasals and frontals are pushed backwards before 
the retiring nostrils. The frontals squeeze the parietals to 
the sides and meet the supraoccipital, whilst the nasals are 
pressed against the front wall of the cranial cavity. Hence 
the “rostrum” represents only the ventral or alimentary 
part of the mammalian facial region, consisting solely of 
the premaxille—which follow the nostrils backwards and 
become very elongated—the maxille, mesethmoid and the 
vomer. 
The maxillz, premaxille and mandibles bear a single 
row of small teeth, very numerous and all of the same size 
‘ (homodont). Each tooth has a single root, and in the 
porpoise is scoop-shaped 
and raised on a short Fig. 376.—TEETH OF PORPOISE x 2. 
base. (In the dolphin 
each is a simple conical 
point.) There are usual- 
ly about twenty-five on 
each side, upper and 
‘lower jaws, and as they 
are homodont we can 
use no dental formula 
but $2 (dolphin $$ to 
$9), There is no suc- 
cession (monophyodont), but there are said to be traces of a 
second or permanent dentition which is only transitory and 
M. : 36 
(From Flower and LyDDEKER.) 
