Willistox.] Mosasaurs. 89 



Of the Mosasaurinse, including the two genera Mosasaurus and 

 Clidastes, the lowest horizon is the upper part of the Niobrara 

 in Kansas. Clidastes ranges into the Fort Pierre, as previously 

 stated by myself. In the eastern Atlantic region this genus is 

 represented by forms closely allied to those from Kansas. Its 

 range, then, is from the upper part of the Turonian through 

 the larger part of the Senonian. 



The typical Mosasaurus is confined exclusively to the Senonian 

 and Danian. Its distribution in North America is reputed to 

 be from New Jersey, Alabama, and Dakota, but some of the de- 

 terminations may be incorrect. The species from the Fort 

 Pierre are, however, clearly congeneric with one or more from 

 New Jersey. In Europe, Mosasaurus is known only from the 

 Upper Senonian and the Danian (upper chalk and Maestrich- 

 tian) ; that is, apparently, from later horizons than those in 

 which the genus occurs in America. 



The two genera Mosasaurus and Clidastes are nearly related, 

 though perhaps sufficiently different to justify their independent 

 existence. 



From the known distribution of the Mosasaurs, Dollo has 

 concluded : 



"Que la Nouvelle-Zeland (ou, niieux, les terres australes) est le 

 centre d'irradiation des Mosasauriens, qui en seraient partis a la fin 

 de l'epoque cenomanienne, auraient vecu uniquement en Amerique 

 durant l'epoque turonienne, auraient emigre" en Europe a l'epoque 

 senonienne et s'y seraient eteints avec l'epoque maestrichtienne." 



The fact that Mosasaurs have been reported from the Ama- 

 zonian Purus, corresponding to the Maestrichtian, would cer- 

 tainly indicate that they had not become at all restricted in 

 distribution in the latter part of their existence. 



The distribution of the Mosasaurs, so far as now known, 

 seems to be of little value in the correlation of the Cretaceous 

 epochs. Only a single genus seems to be of wide distribution, 

 and the nearly related ones may be widely separated in geolog- 

 ical range. Two, perhaps three, distinct types appear suddenly 

 in the Cenomanian, and have continued side by side in the same 

 waters throughout the greater part of the time during which 



