ZOOLOGY. : 223 
The quadratum, cartilaginous in the Anura, is osseous in the 
Urodela, and is obviously represented by a bone beneath the pre- 
operculum of the Dip- 
noi, which, as Huxley 
has shown, is distinct 
from the latter. The 
preoperculum is here 
obviously the squamo- 
sal of Amphiuma and 
other Urodela (fig. 23, . Fig. 23. 
Sq), so that we now have determined the identity-of the reptile 
squamosal with the preoperculum of the bony fish. And, more, it 
Fig. 24. Amphiuma means (from Georgia); profile of cranium. 
" appears to be demonstrated that the squamosal portion of the tem- 
poral bone of the Mammal is the preoperculum of that type.* 
6. On the Homologies of the Colwmelia. 
With regard to the character of the Reptilian columella, the 
following conclusions seem to be indicated by a study of the 
crania of Crocodilus, liystrosaurus, Chelydra, and Iguana. 
There are two modes in which the parietal arch is completed 
laterally among Reptilia. The usual mode among Vertebrates is 
where an alisphenoid connects the parietal and sphenoid bones. 
This characterizes the Crocodilia and Pythonomorpha.t In the 
second mode, the peculiar bone called the columella stands pillar- 
like on the pterygoid, supporting the parietal (in Gecconide not 
reaching the latter). This arrangement characterizes the Lacer- 
tilia, where the alisphenoid is entirely absent. In the Ophidia and 
Testudinata, the parietal sends down a plate-like border or process 
on each side, which in the latter articulates with a flat bone, which 
* The bone homologized by Huxley (Elem. Comp. Anat., figs. 69-78, Sq) with 
the squamosal in the Telosei must, if the above determination be correct, have 
another interpretation. ‘ 
+ The decurved margin of the parietal takes its place in the Ophidia. 
