No. T.] RELATIONSHIPS OF PROTOSTEGA. 29 
The surangular is broad and short, joining the complementary 
by its antero-superior portion. The posterior portion bears an 
articular face, concave from above downwards, and looking 
slightly inwards; the external margin of this face ends on a 
thin ridge. 
The articular is well ossified. Its articular face, slightly 
saddle-shaped, looks backward and upward. The postero-inferior 
part is rounded, and shows largely on the inferior surface of 
the mandible. There is no process on the infero-internal por- 
tion of the articular, as in the Cheloniidae, and this allows the 
condylar face of the angular to curve towards the articular at the 
bottom instead of bending toward the median line of the jaw. 
Plastron. — The hyoplastron is a large, heavy plate, thickened 
in the middle and becoming thin towards the edges, which are 
extended into long, slender, radiating processes. It is roughly 
triangular in shape. The antero-internal portion is bent slightly 
inward, and carried as a strong wing toward the median line, 
where the terminal processes meet those of the same of the 
opposite side. The antero-external edge is smooth, free from 
processes, and concave. The external edge is thickened, and 
the terminal processes are comparatively short and strong. 
The articulation with the hypoplastron was by small and 
numerous closely interlocking processes amounting almost to a 
sutural union. The connection occupied nearly half the posterior 
edge of the bone (Pl. IV). 
The ypoplastron is of almost equal size with the hyoplastron. 
Its general shape is more nearly round, it is furnished with 
processes on the edges, and those on the posterior internal edge 
meet those from the same bones of the opposite side, The 
posterior edge was furnished with two long processes diverging 
posteriorly, between which the xiphiplastron articulated. 
The appended measurements will give an idea of the size of 
these plates, though the loss of the distal ends of the processes 
makes exact measurements impossible (PI. IV). 
Greatest length hyoplastron . . . . . . .649 meters 
ss breadth a 6 tet wo gg = BIRO WIS. |G 
s length hypoplastron. . . . . . .578 
S breadth se Pete: al eee ee 
