224 B. NATURAL HISTORY. 
is in like manner united with the pterygoid* The latter bone is . 
longer than deep (see fig. 9, Col), and very different from the colu- 
mella of Lacertilia, though its position would render it probable 
that it represents that bone. The existence of the parietal or 
squamosal plates in Lystrosaurus, continuous with a veritable 
columella, which rests by a laminiform extremity on the pterygoids, 
confirms the position that the Chelonia, like the Lacertilia, pos- 
sesses a columella. 
In Lystrosaurus (fig. 5, Col), the structure is analogous to that 
of the Crocodilia, already described. The continuity of the inner 
margin of the pterygoids with the presphenoid and sphenoid is 
common to both orders. From a position on this part of the ptery- 
goid, in the genus Alligator, there rises, exactly as in Lystrosaurus, 
an osseous style. (Fig. 18, Col.) It is in front of the foramen 
ovale, and is separated from the alisphenoid by a narrow foramen, 
which opens anteriorly. Above the latter it is codssified in adult 
age with the superior part of the alisphenoid, and both together 
meet the lateral portion of the parietal, which here descends much 
less than in Testudinata, forming only a ridge. I regard the short 
column of Alligator, above mentioned, as functionally the colu- 
mella. 
In the Lacertilia and Rhynchocephalia, the columella is not con- 
tinuous above with any determinable element. (Fig. 12, Col.) 
In the Ichthyopterygia, it is continuous above with the parietal 
extension of the squamosal. (Fig. 3, Col.) 
It appears, therefore, that there are at least four modes of origin 
of the superior extremity of the columella; viz. : — 
Confluent with epiotic or parietal, — Ichthyopterygia, ? Anomodontia. 
Confluent with alisphenoid, — Crocodilia. 
Suturally united with parietal, — Testudinata. 
Approaching or touching parietal without suture, — Lacertilia, Rhynchoce- 
phalia. 
The first two types cannot therefore be homologized with each 
other, nor the second with the third and fourth. The latter two 
forms of columella are probably homologous. 
Having reviewed the homologies of the opisthotic, squamosal, 
and columellar bones, I append a table of their synonymes, with 
those of a few others. 
* This bone is overlooked by Stannius, who says the parietal plates rest on the 
pterygoids. 
