ZOOLOGY. 219 
rates it inferiorly from the exoccipital in several species, the 
superior suture is wanting or invisible. 
The opisthotic is known to be distinct in osseous Ganoids and 
Teleostei. 
5. On the Homologies of the Squamosal Bone. 
As this bone derives its name from its Mammalian representative, 
it will be well to trace it from that class. It may be defined as 
the bone which occupies the space 
between the provtic in front, the opis- 
thotic behind, and the parietal above, 
which subtends the auricular bones or 
meatus superiorly, and forms the poste- 
rior extremity of the zygomatic arch. 
In the Birds the zygomatic arch does 
not exist, and the malleus is produced 
from beneath it, as the os quadratum, 
for the support of the mandible (Par- 
ker).* Here then it first assumes the 
position of the external shield of the 
quadrate, which it continues to hold. 
throughout the series of Vertebrata 
below this point. 
In tracing its homologies in the 
Reptilia, we commence with those in 
which the quadrate is most nearly ses- 
sile on the cranium, as in the Birds, 
and proceed towards those in which 
the latter is supported at the extremity 
of a prolongation of the posterior ele- 
ments of the cranium, or a “suspenso- 
~ rium.” 
I may add here that the former re- 
lation of the quadrate, being most sim- 
ilar to that found in both the Birds and 
the Stegocephalous, and other tailed 
Batrachia, is the most generalized; 
Fig. 18. — Alligator mississippiensis’; cranium, with arches removed, and a pointer passed behind the columella. 
* On the Development of the Skull in the Ostrich Tribe (Philos. Trans., 
London, 1865, p. 118). 
