121 



Tliis motion is permitted by the laminar cliaraeter of" tiu' overlapping margins 

 of" the splenial, etc., as toUows : 



The principal Ijody of the dcntary comes to an ol)tiise but grooved pos- 

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

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

 of this thin plate is nuicli lower than th(; 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 tlie outer, and is concave, turning outward above to conform to 

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

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

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

 onlcr lamina of tlie dcntary, moving liccly on the latter. There is, then, 

 nothing that prevents tliis Irom being a complete articulation, except the 

 lamina of the articular, which is about half a line in thickness, and pnil)al)ly 

 llc.\il)le in lit"e. 



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

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

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

 l)roperly applied to the dentary, and the coronoid and splenial are in line, as 

 they no douljt were under ordinary circumstances in life, the curvature of th(; 

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

 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 

 (his 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 iracture. 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 fuul no 

 provision. 



This structure was no doubt designed to effect the deglutiti(m of large 

 bodies, which would readily pass between tiie expanded mandibular rand. 

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

 IG c 



