48 S. IV. WILLISTON 



ing to modern forms had become well differentiated by the close 

 of the Jurassic, or shortly thereafter, it is not at all unreasonable 

 to suppose that the snakes had branched off as early as, or 

 earlier than, the beginning of the Lower Cretaceous. 



The relationships of the chief groups of the Squamata may be 

 expressed as follows : 

 Order Squamata (Lepidosauria). 

 Suborder Sauria (Lacertilia). 

 Superfamily Platynota. 



Family Varanidae Gray. Pleistocene — recent. 



Genus Varaiius Merrem. India, Australia, Africa. 

 Family Dolichosauridae (Kramb.) Nopcsa. Cretaceous. 



Genera : Dolichosaurus Owen, Acteosaurtcs Meyer, Pontosaurus 

 Kramb., Adriosauriis Seeley. 

 Family Aigialosauridae (Kramb.) Nopcsa. Lower Cretaceous. 



Genera: Aigialosaurns Kramb., Carsosaurus Kornh., Opetiosaurus 

 Kornh., Mesoleptiis Carnalia. 

 Superfamily Mosasauria. 



Family Mosasauridae Gervais. Upper Cretaceous. 



Genera : Mosasaurus Conyb., Clidastes Cope, Platecarpus Cope, 

 Tylosauriis Marsh, Baptosaums Marsh, Plioplatecarpus Dollo. 

 Hainosauriis Dollo, Prognathosanrus Dollo, Phosphorosaurus 

 Dollo, Brachysaiirus Williston. 

 Superfamily Kionocrania (true lizards). Trias — ^ recent. 

 Superfamily Amphisbaenia. Oligocene — recent. 

 Superfamily Rhiptoglossa (chameleons). Recent. 

 Suborder Serpentes (Ophidia). Laramie Cretaceous — recent. 



It is a remarkable fact, for which there has never been any 

 adequate explanation, that the mosasaurs are wholly unknown 

 in the juvenile condition. Altogether, throughout the world, 

 more than three thousand specimens of these animals have been 

 brought to light during the past century, and every one of them, 

 so far as my own knowledge goes — and I have seen many hun- 

 dreds — and so far as anything that has been published would 

 indicate, is supposed to be of an adult or fairly mature animal. 

 Not a single specimen which can be suspected to be embryonic 

 is known. I hardly think that the same can be said of any other 

 group of extinct aquatic animals. In the Cretaceous of Kansas 

 the young of the plesiosaurs are fairly common, perhaps every 

 third or fourth specimen showing immaturity. I have found 



