. Bulletin. Natural history. RUSSELL: AMERICAN MOSASAURS 43 face is a row of teeth commonly bent in a gently recurving line extending from the medical base of the basisphenoid process to the anteroexternal termina- tion of the bone (Tylosaurus, Platccarpus, Prognathodon and Plotosaurus), but these teeth may be oriented in a straight anteroposterior line (Clidastes). The pterygoid teeth are usually smaller, more strongly recurved, and more nearly circular in horizontal section than those of the mandibles (Clidastes, Mosasaurus, Plotosaurus, Platccarpus, Ectenosaurus and Tylosaurus). In Prognath
. Bulletin. Natural history. RUSSELL: AMERICAN MOSASAURS 43 face is a row of teeth commonly bent in a gently recurving line extending from the medical base of the basisphenoid process to the anteroexternal termina- tion of the bone (Tylosaurus, Platccarpus, Prognathodon and Plotosaurus), but these teeth may be oriented in a straight anteroposterior line (Clidastes). The pterygoid teeth are usually smaller, more strongly recurved, and more nearly circular in horizontal section than those of the mandibles (Clidastes, Mosasaurus, Plotosaurus, Platccarpus, Ectenosaurus and Tylosaurus). In Prognathodon and probably also Plesiotylosaurus, however, the pterygoid teeth are not subequal in size, as in the foregoing genera, but increase anteriorly from a very small diameter to equal the mandibular teeth in size. The number of pterygoid teeth varies from a minimum of seven in Prognathodon to a maximum of fifteen in Ploto- saurus. Although the pterygoid must have been firmly buttressed against the palatine, this contact is very rarely preserved. In Platccarpus the main body of the ptery- goid abuts anteriorly onto the medial half of the posterior edge of the palatine and sends a long process anteriorly to cover the ventromedial surface of this bone. In ventral aspect the suture runs from the external side of the pterygo-palatine bar anteromedially in a nearly straight line to meet the internal side of the bar just behind the posterior boundary of the internal naris. Sutural indications on the ventral surface of the palatine in Tylosaurus and Clidastes suggest that the pterygo-palatine contact is probably also similar in these two genera. In Ploto- saurus the suture is the same as in Platccarpus, except that it is more medially directed and reaches the medial edge of the pterygo-palatine bar far behind the internal narial opening. The ectopterygoidal process projects from the main body of the pterygoid at a right angle in Clidastes, Mosasaurus and Tylosaurus and at an acute angl
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