Concerning the origin of the Mosasaurs three altogether different views exist: G. Baur regarded the Mosasaurs as highly specialized aquatic Varanoids; Boulenger is inclined to trace their origin back to the Neocomian Dolichosaurs; and Osborn, in a recent paper, doubts that Varanids and Mosasaurs sprang from a common stem, and regards the latter as “a very ancient offshoot of the Lacertilia.” Some Lacertilia found in recent years in the Lower Cretaceous of Dalmatia, and not yet fully compared with the Mosasaurs, will, I believe, throw fresh light on this subject. Among the fossil Lacertilia found in Dalmatia two types can be distinguished, namely, Dolichosaurs and Aigialosaurs, the former including the genera Dolichosaurus Owen, Pontosaurus Gorjanovic-Kramberger (= Hydrosaurus lesinensis, Kornh.), Acteosaurus Meyer, and Adriosaurus Seeley; the latter, Aigialosaurus G. Kramberger, Garsosaurus Kornh., Opetiosaurus Kornh., and perhaps also Mesoleptos Cornaglia.