121 



This motion is permitted by the laminar character of the overlapping margins 

 of the splenial, etc., as follows : 



The principal body of the dentary comes to an obtuse but grooved pos- 

 terior termination. Its external wall is prolonged more posteriorly, the inferior 

 margin fitting a rabbet of the outside of the splenial. The superior margin 

 of this thin plate is much lower than the truncate extremity ; and its margin 

 gradually rises to meet the outer margin of the latter. The section of the 

 splenial is U-shaped, much thickened at the turn. The inner lamina is more 

 elevated than the outer, and is concave, turning outward above to conform to 

 the dentary. A narrow laminar prolongation of the articular is observed be- 

 tween the folds of the U. An outwardly convex, wedge-shaped terminus of 

 the surangular is included between the inner lamina of the coronoid and the 

 outer lamina of the dentary, moving freely on the latter. There is, then, 

 nothing that prevents this from being a complete articulation, except the 

 lamina of the articular, which is about half a iine in thickness, and probably 

 flexible in life. 



The superior margin of the coronoid is convex outwardly, and is not con- 

 tinuous with that of the dentary, when the elements forming the splenial 

 articulation are in line. When, however, the process of the articular is 

 properly applied to the dentary, and the coronoid and splenial are in line, as 

 they no doubt were under ordinary circumstances in life, the curvature of the 

 upper margin of the ramus is continuous and normal. At the same time, the 

 splenial articulation is strongly flexed, and the inferior outline of the ramus 

 angulate at that point. 



We have in this feature one of the most extraordinary peculiarities of 

 this remarkable order. The mandibular arch, in its usual relations, inclosed 

 a diamond-shaped area, open behind, the portion anterior to the lateral angles 

 the longer, and only closed by ligament in front. The structure is an element 

 of weakness, though, indeed, without such an articulation, such a light and 

 slender jaw would be particularly liable to fracture. There was, no doubt, a 

 strong ligamentous union of the parts, as the grooved adjacent margins testify; 

 but for any supernumerary muscles to flex the dentary bones I can find no 

 provision. 



This structure was no doubt designed to effect the deglutition of large 

 bodies, which would readily pass between the expanded mandibular rami. 

 This lateral extension is necessary to reptiles, which, like the snakes, swal- 

 1G c 



