. Bulletin. Natural history. 16 PEABODY MUSEUM BULLETIN 23 For descriptive purposes the mosasaur premaxilla may be divided into two major portions, a broad anterior tooth-bearing apex and a slender internarial bar. A large cylindrical "prow" projects in front of the premaxillary teeth in Tylosaurus, and a smaller, more conical one is present in Clidastes, Mosasaurus, Ectenosaurus and to some extent in Plotosauriis. The premaxilla terminates directly in front of the most anterior tooth in Platecarpus, Plioplatecarpus (Dollo, 1889b, p. 275), Prognathodon and Plesiotylosaurus. The dorsa
. Bulletin. Natural history. 16 PEABODY MUSEUM BULLETIN 23 For descriptive purposes the mosasaur premaxilla may be divided into two major portions, a broad anterior tooth-bearing apex and a slender internarial bar. A large cylindrical "prow" projects in front of the premaxillary teeth in Tylosaurus, and a smaller, more conical one is present in Clidastes, Mosasaurus, Ectenosaurus and to some extent in Plotosauriis. The premaxilla terminates directly in front of the most anterior tooth in Platecarpus, Plioplatecarpus (Dollo, 1889b, p. 275), Prognathodon and Plesiotylosaurus. The dorsal midline of the. Text-fig. 2. Mosasaur premaxillae. A. Platecarpus iclericus (AMNH 1820, X %). B. Clidastes propython (YPM 1319, X 1/3). C. Tylosaurus proriger (AMNH 1592, X %)• bone may be smooth (Clidastes, Mosasaurus missoiiriensis, Ectenosaurus and Tylosaurus), with a slight dorsal crest (Mosasaurus maximus and Plotosaurus) or slightly sulcate (Platecarpus and Progy^athodon). Paralleling the dorsal mid- line on either side is an irregular row of foramina in Clidastes, Mosasaurus, Plotosaurus, Platecarpus and Prognathodon. In Tylosaurus they are distributed randomly over the sides of the "; These foramina mark the exits of the ophthalmic ramus of the fifth nerve which in Varanus enters the premaxilla near the top of the premaxillo-maxillary sutural surface. In lateral trace this suture varies among the species of mosasaurs, but generally is parabolic in form, rising steeply from the alveolar margin and sweeping posteriorly to merge with the outer edge of the internarial bar. Ventrally the roots of the premaxillary teeth are separated from those of the opposite side by a grooved median ridge (Clidastes, Platecarpus and also in Tylosaurus, according to Williston, 1898b, p. 104). A nubbin on the center of the ventral surface of the "prow" in Tylosaurus is connected by weak ridges to the lateral margin of the dental alveoli and probably marked the ant
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