THE REPTILIAN CLASS OF THE MOSASAUItlD^. 687 



atlantal joint. Above this the exoccipital sends outward a strong 

 process (z'), which, by its short extent, answers to 'the exoccipital, 2, 

 of the tortoise, and terminates freely, not abutting against the tym- 

 panic or other bone, and suggesting the non-development of the 

 paroccipital, 4. A second smaller process (2*) affords attachment to 

 part of the lower border of the mastoid (s) by intervening liga- 

 mentous matter. The superoccipital, 3, develops the diverging 

 penthouse-like ridges, 3', the lower and outer end of which gives a 

 similar close ligamentous attachment to a part of the upper margin 

 of the mastoid (s), opposite the part of the lower margin attached 

 to the exoccipital. Much of the superoccipital is overlapped by the 

 parietal ; but the overlapping stops short of the occipital region, and 

 the seeming bifurcation of the parietal crest is due to the super- 

 occipital. 



The most marked and characteristic modification of the part of 

 the skull here compared is the limitation of the proximal joint of 

 the tympanic, 28, to the mastoid, 8, exclusively. Besides this are to 

 be noted the great length of 8, its free production backward for 

 half that length beyond the occipital plane of the skull, and 

 its liberation from all share in the formation of the otocrane. 

 From this chamber the mastoid is excluded by the suture 

 between the alisphenoid (fig. 4, e) and the parietal 7, over which 

 suture the mastoid, 8, extends in a squamous shape and manner, 

 and so smoothly as to suggest a certain latitude of gliding motion 

 in connexion with the more violent and extensive movements of the 

 columnar tympanic (figs. 13, 28), which is suspended vertically from 

 the free extremity of the mastoid, 8'. I append an outline (fig. 4) of 

 the side view of this part of the skull in a Python, in which the 

 alisphenoid is 6, the parietal 7, the superoccipital 3, the exoccipital 2, 

 the basioccipital 1, its hypapophysis 1', and the stronger continuation 

 of the same on the basisphenoid is 5. The relative position and ex- 

 tent of the mastoid are shown in dotted outline. 



Mosasaurus (fig. 5) shows the laceitian extension and connation 

 of the ex- (2) and par- (4) occipitals, with the expansion and abut- 

 ment of the latter, 4, against the mastoid, 8, and squamosal, 27, — all 

 three bones, and especially the squamosal, combining to form the 

 large articular cavity for the firm junction of the tympanic. 



The superoccipital, 3, completes the great foramen above, has no 

 penthouse ridge as in Python, but is overtopped by the parietal, 

 which forms by its median part, 7, and its long lateral processes, 7', 

 the upper part of the occipital surface. The whole of this surface 

 manifests the lacertian and departs from the ophidian modification 

 of the reptilian skull. 



The parial hypapophyses of the basioccipital, and the outward 

 extensions of the ex- and par-occipitals, are figured in pi. xv. of Prof. 

 Cope's work, in the Mosasauroids which he denominates Platecarpus 

 coryphceus, Plat, ictericus, and Plat, curtirostris. The terminal abut- 

 ment of 2-4 against the mastoid and squamosal is likewise indicated 

 in the figures of cranial fragments in plates xv., xvi., xvii. ; but 

 these figures, like those of the skull of Mosasaurus Hoffmanni, cited 



