Willistox.] Mosasaurs. 131 



nearly horizontal at the extremity. Its upper margin is con- 

 vex, running to the obtuse inferior angle. This much of the 

 bone is the chondrogenous element of the Testudinata and Rhyn- 

 chocephalia known as the articular. The anterior portion, ac- 

 cording to Baur, is the co-ossified angular. "What is the 

 articular of the Lacertilia? A consideration of its relations to 

 the other elements of the mandible teaches that it is nothing 

 else than the chondrogenous articular plus the dermogenous 

 angular of the Testudinata and Rhynchocephalia. The so-called 

 angular of the lacertilians is the splenial, and the so-called 

 splenial is the presplenial of the Chelyoidea." 54 If these con- 

 clusions are correct, it becomes necessary to revise our nomen- 

 clature of the elements of the lacertilian mandible. That the 

 nomenclature here used may be readily homologized with that 

 hitherto used I give the correlative terms, as follows : 



Articular Articular plus angular (articulo- angular). 



Angular Splenial. 



Surangular Supraangular. 



Coronoid Complementary. 



Splenial Prespenial. 



Dentary Dentary. 



That portion of the bone that would be the angular of Baur 

 shows no separation whatever in any of the forms from the 

 articular proper. The suture separating it externally from the 

 surangular proximally and splenial distally runs obliquely 

 downward from the posterior part of the cotylus, and then 

 nearly parallel with the lower border of the bone. Internally it 

 forms a long, thin tongue, extending forward beyond the articu- 

 lation, to be inclosed within a cavity of the presplenial. It is 

 very thin where it crosses the articulation, but is here undoubt- 

 edly capable of bending ; otherwise the joint between the two 

 segments of the jaw would be immovable. Below on the inner 

 side, the anterior projection, the angular proper, is slightly 

 overlapped by the margin of the splenial. Above, it meets the 

 ridge-like convexity of the surangular below the coronoid, in- 

 closing between it and the surangular a long, flattened cavity. 



54 Anatom. Anzeiger, XI, p. 412, 1895, 

 11— IV 



