THE REPTILIAN CLASS OF THE MOSASATTKID^. 703 



sented by a mere tuberosity at the back of the articular surface. 

 In Mosasaurus this process (fig. 18, 29) (" l'apophyse, b, pour le 

 muscle analogue du digastrique," Cuv., p. 820) is relatively as well 

 developed as in Amblyrhynchus (fig. 19, 29) and terminates, as in 

 that Sea-lizard, subacutely ; in Monitor (fig. 20, 29) it is longer, and 

 terminates obtusely. In all these lacertians the process is developed 

 from the " articular " element, not from that called " angulaire " by 

 Cuvier. 



The coronoid process is formed by the " coronoid element," 31', in 

 Mosasaurus as in Monitor and Iguana ; in the Python it is an 

 exogenous lamelliform process of the surangular plate of the 

 angulo-surangular element; to the antero-internal part of this 

 element the coronoid (fig. 21, c) is applied, slightly projecting in 

 front of the coronoid process, and chiefly disposed, like the " sple- 

 nial," to cover the groove at the postero-internal part of the 

 dentary element. 



The " splenial," 33, extends, in Mosasaurus (fig. 18) from the 

 angular, 30, and surangular, 31, elements along the inner surface of 

 the dentary to near the symphysis, as in the Monitor ; in the Python 

 it extends from the angular part of the angulo-surangular to the 

 back part of the dentary, joining the " coronoid " plate above and 

 not extending forward beyond that plate : the length of the entire 

 splenial, here, is but one fourth that of the entire mandibular 

 ramus. This small splenial plate in Ojphidia is imperforate ; in 

 Mosasaurus it is perforate*, as in Monitor and Iguana. In Python 

 the outer plate of the dentary (fig. 21, 32) is deeply notched 

 behind by a long angular depression which receives a process of 

 similar shape of the angulo-surangular element, 31. In Mosa- 

 saurus, as in Monitor, the outer plate of the dentary, 32, terminates in 

 a subvertical line ; this is curved in Iguana, less so in Monitor, still 

 less in Mosasaurus f, where it seems to have suggested to Prof. 

 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 s s, on the outer 

 side of the ramus, and in a greater degree upon the inner surface 

 of the ramus, must have as effectually opposed such flexion in Mosa- 

 saurus as is the case with Lacertians and, a fortiori, with Ophidians. 



The movements of the mandible in serpents, and especially in 

 Pythons and Boas, in relation to the engulfiing of large prey, are 

 chiefly subserved by the absence of a " symphysis " or anterior arti- 

 culation of the right and left branches of the mandible. These are 

 here connected to one another by elastic ligaments. 



In Mosasaurus the relative extent of the symphysial joint of the 

 mandible is greater than in existing Lizards, in which the symphysis 

 is completed b}' inelastic or unstretchable fibro- cartilage. 



In Mosasaurus Hoffmanni the outer plate of the dentary is pierced 

 by about a dozen holes in a regular longitudinal series. There are 

 six or seven corresponding foramina in the Monitor (fig. 20), and 



* "Il'y avoit une petite ouverture dans l'operculaire." Tom. cit. 



t " Le sur-angulaire se joignant carrement avec le cientaire." Tom. cit. 321 . 



