. Bulletin. Natural history. RUSSELL: AMERICAN MOSASAURS 35 the anterior two-tliirds of its length, then rises at an angle of 45° to the vertical posterior end of the bone, and abuts posteroventrally against the basioccipital. A rounded pit located at the posterodorsal corner of the internal surface of the. Text-fig. 13. Medial view of braincase of Clidastes propython (VPM 1368, X 1). prootic contains the exits for the undivided stem of the seventh nerve antero- ventrally and those for the two branches of the eighth nerve dorsally. A slight prominence descends anteroventrally from the top of t
. Bulletin. Natural history. RUSSELL: AMERICAN MOSASAURS 35 the anterior two-tliirds of its length, then rises at an angle of 45° to the vertical posterior end of the bone, and abuts posteroventrally against the basioccipital. A rounded pit located at the posterodorsal corner of the internal surface of the. Text-fig. 13. Medial view of braincase of Clidastes propython (VPM 1368, X 1). prootic contains the exits for the undivided stem of the seventh nerve antero- ventrally and those for the two branches of the eighth nerve dorsally. A slight prominence descends anteroventrally from the top of tlie bone in front of this pit, then curves posteriorly to merge with the inner wall of the prootic. The anterior vertical semicircular canal intersects the cartilaginous surface of the supraoccipital suture above and somewhat behind the posterior apex of the prootic incisure. The posterior edge of this suture is strongly emarginated near its center for the ascending sinus utriculus. Camp (1942, fig. 19) illustrates a tiny foramen for the endolymphatic duct lying in front of and medial to this emargination in Platecarpus. The anterior part of the vestibular cavity itself is very similar to that of Varamis (see Bahl, 1937, fig. 10). The posterior branch of the acoustic nerve opens into the utriculus just above the cochlear crest and anterior to the prootic- opisihotic sutine. The anterior branch of this nerve enters the anterior ampul- lary recess which contains the horizontal and anterior ampullae. The horizontal and anterior vertical semicircular canals diverge from their respective ampullae, the former laterally and the latter anteriorly. The cochlear lagena is divided into ecjual halves by the prootic-opisthotic suture; it is conical in form with a pointed ventral terminus. A slight ridge separates the internal auditoi7 meatus from the utricular cavity and marks the site of the fenestra ovalis. The prootic of the following forms is described only as it is known to differ fro
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