COPE ON OWEN ON PYTHONOMOKPHA. 301 



Another question of fact is raised in regard to the possibility of the 

 lateral horizontal flexure of the mandibular ramus in the various genera 

 of Fythonomorpha. My critic states, — "In .Python the outer plate of 

 the dentary is deeply notched behind by a long angular depression 

 which receives a proces's of similar shape of the angulo-surangular 

 element. In Mosasurus as in Monitor, the outer plate of the dentary 

 terminates in a subvertical line j this is curved in Iguana, less so in 

 Monitor, still less in Mosasaurus, where it seems to have suggested to 

 Professor Cope the idea of a movable articulation with the hinder part 

 of the ramus : but the relative overlapping position of the mandibular 

 elements, causing the angular break of the line" [of the posterior border 

 of the dentary] "on the outer side of the ramus, and in a great degree of 

 the inner surface of the ramus, must have as effectually opposed such 

 flexion in Mosasaurus, as is the case with Lacertians and a fortiori with 

 Ophidians." 



I have not had the opportunity of studying a perfect mandibular 

 ramus of a species of the genus Mosasaurus; but I have numerous 

 mandibles of Platecarpus, Liodon, and CUdastes. In all of these, the mo- 

 bility is indicated by the character of the adjacent extremities of the 

 segments of the lower jaw, as well as by the form of the proximal end 

 of the OS quadratum, by which that jaw is mediately articulated with 

 the skull. There is no " overlapping of the mandibular elements causing 

 the angular break" in these genera, either in the horizontal or vertical 

 lines, although the inferior portion of it, where the ball-and-socket artic- 

 ulation is found, forms a slight angle with the remaining portion of the 

 hinge. The anterior extremities of the surangular and coronoid are con- 

 tracted to an obtuse edge, which fits into a groove or rabbet of the den- 

 tary and splenial elements, so as to form a movable joint, the two 

 segments of the ramus being held together by a lamina of bone which 

 in life was doubtless perfectly flexible. This flexure is rendered neces- 

 sary when the jaw is opened widely by the form of the proximal end of 

 the OS quadratum. This extremity forms a sliding joint with the inferior 

 face of the opisthotic; and as it is bent or curved in form, its movement 

 necessarily causes a rotation of the quadrate round its vertical or long 

 axis. This rotation of course throws the proximal part of the mandibular 

 ramus outward; and to permit this movement, the joint near the middle 

 of the latter is clearly adapted. The degree of flexure is dependent on 

 the degree of rotation, and that in turn on the curvatui-e of the prox- 

 imal end of the quadrate. This curvature depends on the development 

 of the " proximal internal angle", which is very large in CUdastes and 

 lAodon, and smaller in Mosasaurus. It is possible that the power of flexure 

 was small in the latter genus, and that Professor Owen's conclusions in 

 the matter may be due to imperfect material. 



Under the head of matters of fact may be mentioned a few points in 

 the history of the discovery of the structure of the Pythonomorplia. I 

 have claimed in my work that the discovery of the hind limbs and much 



