ZOOLOGY. 215 
In Mammalia it is confluent with the elements mentioned, re- 
maining distinct from the exoccipital, and forming part of the 
“mastoid and petrous portions of the temporal.” (Huxley.) 
In Aves it is early confluent with the exoccipital. (Parker.) 
In Reptilia it is distinct in all the orders except the Crocodilia, 
where it is confluent with the exoccipital. (Fig. 11, Exo.) 
This group resembles the higher vertebrates in the close union 
of the quadratum with the prodtic and other cranial bones; and we 
pursue the line of extreme Reptilian divergence in following the 
gradual removal of the quadrate from the cranial walls, on the 
Fig 11. — Alligator mississippiensis ; cranium from behind. 
extremity of a suspending cylinder, which reaches its highest 
expression in the Ophidia. First in this succession comes the 
separation of the opisthotic. 
We have already seen its position in Ichthyopterygia (fig. 1, 
Opo) where it is peculiar in separation from the supraoccipital 
and connection with the basioccipital We have also seen an 
element in the Anomodontia identified with it (fig. 6, Opo) which 
differs in its connections, by being attached to the supraoccipital 
and exoccipital only. 
Passing to the Testudinata, the element maintains the same con- 
nections, with the addition of that with (fig. 10, Opo) the prodtic 
anteriorly, and is extended externally over the proximal extrem- 
ity of the quadratum, a connection not observed in the types 
just described. 
