SKELETON. 71 
groups vacuities or fossz appear in the postero-lateral parts, these being 
bounded by bars or arcades of bone. At most there may be three of 
these fosse. The more lateral of these, the infratemporal fossa 
(fig. 68), is bounded laterally by the zygomatic and quadratojugal, 
while on the inner side it is separated from the supratemporal fossa 
by a -squamoso-postorbital arcade. The posttemporal fossa lies 
between parietal, supratemporal and occipital bones. Occasionally 
only the infratemporal fossa is present, or, by disappearance of the inter- 
vening arcade, infra~ and supratemporal fosse may unite in a single 
temporal fossa. Lastly, by the breaking down of the zygomatic- 
postorbital bar, the temporal fossa and the orbit may unite. 
One or another of these bones may disappear in some groups, either by fusion 
or by complete dropping out. Occasionally they may obtain different connexions 
and relations as in the case of the quadrate in mammals (see ear bones) so that the 
homologies are traced with difficulty. The complexity is increased by the fusion of 
membrane bones and cartilage bones and by the union of cranial bones with those 
of the visceral arches. 
In the lower jaw there are no such extensive modifications as in the 
upper. At most Meckel’s cartilage gives rise by ossification to two 
bones in either half. Behind, at the articulation of the jaw with the 
quadrate, there is an artic- 
ular bone, while at the 
tip, at either side of the 
union (symphysis) of the 
two halves of the jaw, there 
is rarely a mento-Meck- Fc. 69.—Reconstruction of developing jaw of Scele- 
lian hone. ‘ihe ast ot porus, cartilage dotted; letters as in fig. 68. 
Meckel’s cartilage forms an axis around which the membrane bones 
which form the definitive jaw are arranged. These are, at most, as 
follows: (1) a dentary which surrounds the Meckelian in front and 
usually bears teeth; (2) a splenial on the inner side, behind the 
dentary and frequently bearing teeth; (3) an angulare on the lower 
side, usually extending back to the hind end of the jaw; (4) a supran- 
gulare on the outer posterior part of the jaw; (5) a coronoid on the 
upper side, affording attachment for the muscles which close the 
jaws; and (6) a goniale (dermarticulare) on the medial and ven- 
tral sides of the articulare, with which it usually fuses. This whole 
series is present in few vertebrates, dentary, splenial and angulare 
being the most constant. 
