. Bulletin. Natural history. " , &i^'^'^ f^y Text-fig. 5. Prefrontal. A. Tylosaurus proriger (AMNH 4909, X %). B. Platecarpus ictericus (AMNH 1820, X 1/2). anteromedially into an exposed diamond-shaped surface; 2) a triangular lateral plate bounded by the maxilla anteroventrally, by the orbit posteriorly, and by the dorsal surface of the skull above; 3) a vertical wall of bone descending from, the anterior limit of the supraorbital process. The supraorbital process of the prefrontal is quite variable in mosasaurs. In Clidastes and to a lesser extent in Mosasaurus, Prognathodon, Plotos
. Bulletin. Natural history. " , &i^'^'^ f^y Text-fig. 5. Prefrontal. A. Tylosaurus proriger (AMNH 4909, X %). B. Platecarpus ictericus (AMNH 1820, X 1/2). anteromedially into an exposed diamond-shaped surface; 2) a triangular lateral plate bounded by the maxilla anteroventrally, by the orbit posteriorly, and by the dorsal surface of the skull above; 3) a vertical wall of bone descending from, the anterior limit of the supraorbital process. The supraorbital process of the prefrontal is quite variable in mosasaurs. In Clidastes and to a lesser extent in Mosasaurus, Prognathodon, Plotosaurus and Plesiolylosaurns, there is a broad triangular ala projecting laterally from the supraorbital process. This ala is present only as a small nubbin in Platecarpus and Plioplatccarpus (Dollo, 1889b, pi. 9, fig. 6) and is entirely absent in Ecteno- sauriis and Tylosaurus. In Clidastes and possibly also Globidens the prefrontal and postorbitofrontal do not come in contact beneath the frontal. In Prognatho^ don these bones contact, and in Platecarpus and Mosasaurus the postorbito- frontal underlies a small portion of the posterior tip of the prefrontal. In Tylosaurus the supraorbital process of the prefrontal receives a tongue from the postorbitofrontal in a posteriorly deepening groove in its lateral margin. These two elements are nearly fused in Plotosaurus. SUPERCILIARY There is an element infrequently associated with Platecarpus skulls that can only be identified as the superciliai7. It is, like the ectopterygoid, an "L"-shaped bone with a broad short wing p>ossessing an oblique sutural surface on its dorsal (?) face and a longer and slenderer wing whose lateial edge is beveled off. There is no sharp, anteriorly projecting point at the anterior apex of the "L" as- is usually the case in Varanus. What is here taken as the ventral surface is smooth and longitudinally concave. Both Baur (1892, p. 15) and Camp (1942, p. 32) have noted a roughened area
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