32 Field Columbian Museum — Geology, 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 slight 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 PI. V the mandi- 

 ble oiSphcnodon, Crocodiltts, Clwlyd/a, Vara/uts, 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 mare firmly ossified bones of Clidastes 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. 



Vertebr/e. — Atlas and axis. (PI. XXII.) The atlas 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 



