the frog from the side and behind; b.c. is the brain-case, o.c. the
otic capsule, e. the eye, n.c. the nasal capsule, p.p. the
palato-pterygoid cartilage, mx. the maxillary membrane bones, sq.
the squamosal, and mb. the mandible. The student should compare
with Figure 5, and convince himself that he appreciates the
diagrammatic rendering of these parts. Now all the distinctive
differences in form, from this of the dog's skull (D.), are reducible to
two primary causes--

(1) The brain is enormously larger, and the brain-case is vastly
inflated, so that--

(a) the otic capsule becomes embedded in the brain-case wall;

(b) the palato-pterygoid rod lies completely underneath the
brain-case instead of laterally to it;

(c) the squamosal tilts down and in, instead of down and out,
and the lower jaw articulates with its outer surface instead
of below its inner, and, moreover, with the enormous distention
of the brain-case it comes about that the squamosal is
incorporated with its wall.

(2) The maxilla anteriorly and the palatine posteriorly send down
palatine plates that grow in to form the bony palate, cutting off
a nasal passage (n.p.) from the mouth cavity (m.p.), and carrying the
posterior nares from the front part of the mouth, as they are in the
frog, to the pharynx. Hence the vomers of the dog lie, not in the
ceiling of the mouth, but in the floor of this nasal passage.

Section 31. The quadrate cartilage of the frog is superseded by the
squamosal as the suspensorium of the lower jaw. It is greatly
reduced, therefore; but it is not entirely absent. In the young mammal,
a quadrate cartilage can be traced, connected with the
palato-pterygoid cartilage, and articulating with Meckel's cartilage. Its
position is, of course, beneath the squamosal, and just outside the
otic capsule. As development proceeds, the increase in size of the
quadrate, does not keep pace with that of the skull structures. It loses
its connection with the palato-pterygoid, and apparently ossifies as a
small ossicle-- the incus of the middle ear. A small nodule of
cartilage, cut off from the proximal end of Meckel's cartilage, becomes
the malleus. The stapes would appear to be derived from the hyoid
arch. Hence these small bones seem to be the relics of the discarded
jaw suspensorium of the frog utilized in a new function. Considerable
doubt, however, attaches to this inte