THE KEPTILIAN CLASS OF THE MOSASAUKID^. 701 



the dentary, 32 ; but this unites with the surangular, 31, and a small 

 part of the coronoid, 31', as in the Monitors. In all these Lacertians 

 the increasing thickness of the ramus toward its lower border 

 augments the breadth of that part of the subvertical suture. But 

 the sutural line, s s, in each instance describes a more or less 

 zigzag or angular course, impeding transverse yielding or move- 

 ment of the fore upon the hind half of the ramus. 



Definition of the constituent elements is clear in the perfect 

 mandibles of Mosasaurus Hoffmanni and M. Maximiliani, in which 

 latter they bear the Cuvierian letters in Prof. Goldfuss's mono- 

 graph*. 



In the least-mutilated of the subsequently discovered American 

 mandibular fossils, ascribed by Prof. Cope to his genus Clidastesf, 

 the sutures, though the pieces they join are not lettered, are 

 sufficiently traceable to afford sure ground of comparison with the 

 Lacertian type of mandible on the one hand and the Ophidian type 

 on the other. The vertical joint between the angular and splenial 

 is unmistakably defined, and, with the indications of the other 

 sutures, show, as in Mosasaurus Maximiliani (fig. 18) the Moni- 

 torial, not the Iguanian pattern. 



All the portions of Mosasaurian mandibles which are figured by 

 Prof. CopeJ, with literal references to the constituent pieces, afford 

 ample means of deciding the affinities of the fossils in the Reptilian 

 series, as such affinities are exemplified in this guiding part of the 

 skull. 



Of the bones of the Maestricht specimen described and figured by 

 Cuvier (torn, cit.), and the type of the genus Mosasaurus, the most 

 complete is the mandible. The notes and comparisons by the great 

 palaeontologist, by which the Lacertian affinities, as against the Cro- 

 codilian and Cetacean ones are illustrated, extend over three pages 

 (319-322) ; they are a model for his successors. My additional 

 evidences confirm (or, 1 should say, accord with), the characteristics 

 assigned by Cuvier to the " machoire inferieure bien entier," from 

 Maestricht § ; and there remains only, in relation to the present aim, 

 to add a few comparisons of the Mosasaurian mandible with that 

 in the Ophidia. In Mosasaurus (fig. 18) the tympanic articular 

 surface is excavated chiefly in the surangular, 31, and is simply 

 concave, corresponding to the convexity of the tympanic condyle. 



In Ophidia (Python, fig. 21) the articular surface is excavated in 

 the one bone, 31, formed by confluence of the articular, the angular, 

 and the surangular elements : the surface is " ginglymoid," the 

 transverse convexity equalling the longitudinal concavity. 



The angular process is almost obsolete in Python, being repre- 



* Op. cit. Taf. vii. 



t Tom. cit. pi. xiv. figs. 1 and 2. 



I Plate xxii. figs. 3, 4 ; pi. xxvi. figs. 2, a, b, c ; pi. xxviii. fig. 2. 



§ I have only to remark that, in pi. xviii. fig 1. the letter x, indicating the 

 " surangular" in the Monitor (ib. fig. 5) and Iguana (fig. 6), is placed upon the 

 " operculaire " (my "splenial") in the Mosasaur (fig. 1). The references 

 (p. 321) to " figs. 2 et 3," should be to "figs. 4 et 5," in pi. xviii. 



