ZOOLOGY. 207 
which I suppose to be the prodtic. From the anterior and more 
horizontal portion of the pterygoid expansion, a thin laminar bone 
rises, which presents an angle outwardly. Superiorly and inwardly 
it appears to be continuous with a slender prolongation of the 
anterior angle of the parietal plate already mentioned. Not 
suspecting its existence, I destroyed a portion of this rod, in re- 
moving the matrix; but a piece from a point intermediate between 
the parietal and pterygoid extremities remains attached to the 
specimen in place. This element is, no doubt, the columella, 
whose existence in this group of Reptilia has not heretofore been 
suspected. It encloses a narrow vertical antero-posterior foramen 
with the presphenoid. 
Two openings into the brain-case are visible: that between the 
parietal plates, common to most Reptilia, and the foramen, trans- 
mitting the fifth cranial nerve, the combined foramina ovale and 
rotundum. Another foramen is enclosed between the pterygoid 
and the element which bounds the proétic in front and below. A 
narrow bone with rounded edge extends from the superior origin of 
the columella, downwards and outwards to the prodtic, bounding 
the foramen ovale above. It resembles the rod-like projection of 
the columella of Testudinata (see fig. 5), but that is below, not 
above, the foramen. 
The exact composition of the suspensoria of the os quadratum 
is a little difficult to determine, owing to the obscurity of the 
sutures. The posterior parietal arches (fig. 7, Pa) are narrow and 
short, the posterior boundaries of the temporal fossa being chiefly 
formed by the squamosals. The latter commence on each side of 
the parietals, a little behind the anterior extremity, and form the 
overhanging margin of the temporal fossa, inwardly as well as 
posteriorly. The posterior plate of the parietal on each side is — 
proximally enclosed between the squamosal and supraoccipital, 
then between the former and a thin laminiform bone, which 
extends laterally from the supraoccipital, and above the exoccipi- 
tal. It is in contact with the squamosal for most of its length, 
but does not extend to opposite the zygomatic arch, and of course 
not to the os quadratum. This element, in spite of its exclusion 
from articulation with the quadratum, appears to be homologous 
with that which in Iguana extends from the same position to that 
articulation, and which is evidently homologous with the opisthotic 
of the Testudinata. 
The squamosal is very largely developed in Lystrosaurus. Con- 
