THE REPTILIAN CLASS OF THE MOSASAURIMC. 



691 



toid surface is 4 inches 2 lines ; that of the distal, or mandibular, 

 surface, is 4 inches 3 lines. 



The proximal articular surface, fig. 11, a, is in great part rough 

 for close ligamentous or fibro-cartilaginous union with the mastoid 

 and squamosal : it is convex antero-posteriorly, curving downward 

 and backward, as in Amblyrhynchus, leaving the fore part of the 

 proximal end of the tympanic free and smooth. Transversely the 

 rough articular surface, a, is flat or feebly concave. Any swinging 

 movement of the tympanic seems not to be permitted by this joint. 

 The fore part of the tympanic is a broad plate of bone forming 

 that wall of the large tympanic cavity, fig. 9, c : the margin of 



Fig. 9. 



Fig. 10. 



Outer surface, Inuer surface. 



Tympanic bone of Mosasaurus Hojpmanni, ^ natural size. 9' and 10', the same 



bone of Python Sebce. 



the plate, b, which is turned outwaid (laterad), is deeply grooved, 

 as in Amblyrhynchus, for the attachment of the drum-membrane. 

 The tympanic cavity, c, is vertically oblong, 5 inches in that 

 direction across the outlet, 3 inches transversely across the upper 

 end, and gradually contracting to 1| inch as it descends; its 

 depth is 2 inches at the deepest part. The stapedial orifice, d, 

 is at the upper and back part of the cavity, and is of a subelliptic 

 form, 9 lines in long diameter. The proximal articular end arches 

 over this aperture to within a line of the substapedial tuberosity, e. 

 From the lower end of the grooved part of the frame the border 

 curves backward and upward, with a sbarpish ungrooved margin 

 terminated abruptly by a notch, /, within an inch of the end of the 

 upper arched border contributed by the mastoid articular surface. 



The hind part of the inner wall (fig. 10) of the tympanic cavity is 

 transversely convex where it is continued backward into the stape- 

 dial foramen and upon the substapedial process. 



