232 
PROFESSOR H. G. SEELEY OR THE STRUCTURE, ORGARIZATIOX, 
increases from 8 millims. to 2 centims. in passing- inward to form the thickened wall of 
the cerebral cliamher. There is some indication tliat this thickening may be due to 
another bone, the pro-otic, resting in front of the ex-occipital, with which it is no^^' 
closely blended, for a transverse fracture on the right side appears to show a nearly 
obliterated suture, which extends upward from the articular cup in the hypapophysis over 
the lateral plate by an oblique chanjiel which coincides with the thickening of the plate. 
The Bach of a Skull ivhich shoivs the whole of Brain-case and the Relation of the 
Quadrate Bone to the Pterygoid and Squamosal. (Plate 12.) 
The British Museum specimen (B. 866) comprises much of the back of a skull 
posterior to the parietal foramen, and indicates a new species. The squamosal bones 
are directed backward, so that the part of the squamosal and occipital plate which is 
anterior in the last specimen is lateral in this. Tlie scjuamosal bone has its vertical 
contour convex posteriorly, and concave distally. The transverse extension of the 
bone extends vertically above the level of the post-parietal region of the brain-case, 
which is convex in length as well as transversely. The quadrate bone passes under 
an arch in the squamosal, so that its posterior and articular part is hidden under that 
bone. The pterygoid bone is vertically compressed posteriorly, so as to form a sharp 
ridge on the palate. The bones above the brain-case form a narrow vertical plate 
which expands transversely. In this species the atlas and axis are anchylosed. 
Every character separates it from the other described species. It may be named 
Dicynodoji [Tropidostoma) Dunnii. 
The squamosal plates, which diverge backward and outward, are remarkably convex 
from above downward. They approximate superiorly, so that the transverse width 
over the middle of the concavities in which they terminate is a little over 5 centims. 
Posteriorly their mutually inclined surfaces are separated by a wedge-shaped vacuity, 
which is 2-| centims. wide superiorly, and extends forward between the roof bones of the 
skull for more than 5 centims. (fig. 4), becoming a mere groove in front (fig. 1) which 
can be traced along the median suture. The transverse width posteriorly in the middle 
height of the squamosals is over 11 centims., while the measurement over the articu¬ 
lation for the lower jaw is 7 centims. Hence the squamosal bones enclose an oval 
basin-shaped excavation at the back of the head, where the atlas and axis and suc¬ 
ceeding cervical vertebrre are attached. The specimen gives no indication of post¬ 
mortem compression. The condition of the quadrate bone is unusual. It commonly 
lies in front of the distal end of the squamosal bone, and forms a flattened wedge, 
convex on its superior border, concave posteriorly ; here it contracts distally to form 
the long nariow ovate convex condyle, which is directed oblicjuely forward, and 
crossed by a slight longitudinal groove. Each condyle is about 3 centims. long 
(though the left appears to be slightly shorter), and anteriorly measures 1-3 centim. 
transversely. The condyles are entirely hidden from side view by the squamosal 
