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