692 



PROP. OWEN ON THE RANK AND APPINITIES IN 



This convexity indicates the columnar part of the tympanic, which 

 undergoes, as it were, a half twist to form the distal or mandibular 

 articulation, the long diameter of which is transverse to the skull's 

 axis, instead of being, as at the upper end of the bone, sub- 

 parallel therewith. The long diameter of the distal joint, fig. 12, 



Fig. 11. 



Fig. 12. 



Proximal end. Distal end. 



Tympanic bone of Mosasaurus Hoffmanni, X \. 



is 4 inches, the short diameter is 1 inch 9 lines at the middle 

 part, narrowing to each end, and chiefly to the inner (mesial) end. 

 The articular surface, fig. 12, g, is convex both lengthwise and 

 across, and smoother than the upper surface. It is not concavo- 

 convex for a ginglymoid joint, as in Python, but forms the ball 

 of a simple ball-and-socket joint. There is a tuberosity, fig. 10, Jc, 

 for the ligamentous attachment of the end of the pterygoid behind 

 and above the mandibular articular surface. 



The fore part of the tympanic is chiefly formed by the tympanic 

 plate, which there shows a broad, smooth, slightly sinuous surface ; 

 the hind part is chiefly formed by the deflected portion of the mastoid 

 surface above, and by the mandibular surface below. The inner (me- 

 sial) side of the bone is the narrowest, and shows a smooth elliptic 

 cavity, fig. 10, i, with a sharp border, 10 lines by 7 lines in outlet 

 diameters and 5 lines in depth : the long axis is subvertical : from 

 the lower end of the cavity a subobtuse ridge, Jc, is continued 

 downward for lg inch, ending abruptly. It is possible that this 

 cavity may have received the end of a long outstanding parocci- 

 pital process, as in the Lacertilia ; but I have not found a corre- 

 sponding cavity in the recent skulls of the order which I have 

 examined. There is no trace of such in Python, nor any abutment 

 of paroccipitals, or produced exoccipitals, against the proximal part 

 of the tympanic, as in Lacertilia. 



I have noted a difference in the size and shape of the stapedial 

 outlet of the tympanic cavity in tympanies of similar-sized Mosa- 

 saurs from the Maestricht Chalk, but not in a degree countenancing 

 specific, much less, generic, distinction. 



In the Mosasauroid referred to Clidastes tortor by Prof. Cope 

 the proximal surface arches down to a somewhat lower part of the 

 tympanic than in Mos. Hoffmanni, and the substapedial extremity 

 is nearer the distal articular surface. 



