DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
125 
The front pair, or olfactory capsules, are half the length of the skull (fig. 17, al.n., 
al.e .); the hind pair, or auditory (chi.), take up much of the skull in its hinder part, 
aborting, by their implantation, a large amount of the inferolateral walls in front of 
the occipital arch. The roof is so largely open, when the investing bones have been 
removed, that the sides and base can be studied as well from the upper, as from the 
under, face. 
On the under face of the skull (fig. 1)1 have figured three bones of the vomerine 
series, although they belong to the investing bones ; this is because of their peculiar 
relation to the olfactory capsules, and especially the parts called “Jacobson’s organs.” 
Beginning at the snout (fig. 1, al.n.), we see that the external nostrils (e.n .) 
are at present inferolateral in position, being seen better on the lower (fig. 1) than 
on the upper face (fig. 2). Their direction is oblique, and they are extensive open 
spaces. The rounded and emarginate fore end of the snout is followed by the coils 
that surround the nostrils, which widen out and are marked off by a groove ; behind 
these coils, the snout enlarges on the under surface into two large flaps, that meet 
in the middle at an obtuse angle, which, however, is cut away, so to speak, and made 
acute by the sinuosity of the selvedge of the flaps. Here the labyrinth has its walls 
pinched in, before it expands to form the swollen olfactory or ethmoidal region. The 
margining hind flaps of the snout are only partially free, yet they overlap (or rather 
grow under) the two pairs of cartilaginous growths into which they are developed, 
backwards. The outer pair of cartilaginous growths simply form the general wall of 
the capsule, here called aiiseptal (al.sp.); the sides of the capsule which are con¬ 
fluent with the septum nasi, above (fig. 2), are tucked under, below ; and inside their 
edges another tract of cartilage is seen of, apparently, the same width as this arrested 
floor. This submarginal tract is the inferior turbinal (i.tb.) and is really very exten¬ 
sive, as we shall see in the sections (Plate 18); it arises as a longitudinal outgrowth 
from the inner face of the outer wall, and is half the length of the entire labyrinth 
There are also two submesial cartilages, three-fourths the size of the inferior 
turbinals; these are retral developments of the snout, and I call them simply the 
“recurrent cartilages” (rc.c.) ; they are, however, very important, being the proper 
capsules of Jacobson’s organs. These tracts only partially close in above, forming a 
sort of trough in which the organs of Jacobson lie. They are elegant, somewhat 
sigmoid, long, revolute leaves of cartilage, with their convex face looking outwards 
and downwards. 
Each leafy part is supported by a bone, the form of which they dominate, so that 
each tract is also hollow on the face that looks towards the curved inner edge of the 
cartilage; it lies on the inside, back to back to its fellow : these are the front, paired 
vomers (v'.), and answer to the paired vomers of the Snake and Lizard among 
Reptiles.* 
* In the two latter these paired vomers are very curiously modified, and have over them an extra pair 
of bones (septo-maxillaries), the two bones on each side forming a capsule to the organ of Jacobson, 
