32 FIELD CoLuMBIAN MustuM—GEo Loecy, VoL. II. 
the angular of Baur. It lies above the splenial, disappearing beneath 
the coronoid anteriorly. Posteriorly it is joined by a suture with the 
articular, approaching but not quite entering into the cotylar surface, 
or if so, only to a shght extent. This end has been dislodged slightly 
from its normal position and is slightly twisted upward. It is 
scarcely possible that this is due to fracture, since the surface has all 
the indications of a suture, and a fracture could hardly have occurred 
here without injury to the bone underneath. The end is slightly 
thickened and fits into a pit on the anterior upper part of the articu- 
lar rim; just below the suture, separating it from the articular, there 
is a longitudinal ridge-like roughening, and a narrow, deep pit. This 
element I call the prearticular. 
For the sake of comparison, I have figured in Pl. V the mandi- 
ble of Sphenodon, Crocodilus, Chelvdra, Varanus, with the interpretation 
of the elements as here accepted. 
The bones of the skull, as of the entire skeleton, seem to have 
had a sort of postmortem plasticity. Apparently during life the 
sutures everywhere were free, and the parts all readily separable, and 
wherever the bones have been disturbed or distorted the sutures have 
pulled apart and widened. Where there has not been such disturb- 
ance, however, the sutures are often obliterated, the elements fusing 
together. This would seem to indicate youth, but plasticity in the 
Cretaceous skeletons was largely due to the composition of the bones, 
which may have been more or less persistent throughout life. Those 
in which the inorganic proportions were large have suffered less from 
postmortem disturbances than those in which the organic material was 
considerable. Bird bones were never plastic, and very rarely are the 
bones crushed, the cavities being filled with crystalline material often. 
Of the pterodactyls, however, the bones are invariably found crushed, 
though presenting little evidence of plasticity. Among the mosasaurs, 
the more firmly ossified bones of C/dastes are less often changed in 
shape, while the Tylosaurs, on the other hand, were more or less 
subjected to a plastic distortion. The structure of the plesiosaur 
bones in all that I have seen is unusually soft. | 
VerTEBRZ.—AZ¢/as and axis. (PI. XXII.) The a¢/as has the 
usual number of elements, the intercentrum and the two side pieces, 
or neurapophyses. It will be convenient, however, to describe in 
this connection the parts of the whole axial and atlantal complex, that 
is, in addition to the odontoid, the axial intercentrum and the axial 
centrum and arch. The arrangement of all these parts is very like 
that in the lizards, crocodiles and various other reptiles, save that the 
