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WILLISTON: RANGE AND DISTRIBUTION OF THE MOSASAURS. b79 
Of the Mosasaurine, including the two genera Mosasaurus and 
Clidastes, the lowest horizon is the upper part of the Niobrara in 
Kansas. C/idastes ranges into the Ft. Pierre, as previously stated 
by myself. In the eastern Atlantic region his 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 dAZosasaurus 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 determina- 
tions may be incorrect. The species from the Ft, 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 Mestrichtian), 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 sufficiently different to justify their independent existence. 
From the known distribution of the Mosasaurs, Dollo has con- 
cluded, ‘Que la Nouvelle-Zéland (ou, mieux, les terres australes) 
est le centre d’irradiation des. Mosasauriens, qui en seraient partis 
a la fin de l’époque cénomanienne, auraient vécu uniquement en 
Amérique durant l’époque turonienne, auraient émigré en Europe 
a ’époque sénonienne et s’y seraient éteints avec l’époque mestr- 
ichtienne.” The fact that Mosasaurs are reported from the Ama- 
zonian Purus, corresponding to the Mestrichtian, would certainly 
indicate that they had not become at all restricted in distribution 
in the latter part of their existence. 
The distribution of the Mosasaurs, so faras 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 geological 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 the group has been in exist- 
ence. Some minor divergent forms have appeared, such as the 
singular Phosphorosaurus Dollo, Prognathosaurus and Brachysaurus 
and, perhaps, Baplosaurus Marsh, which, by the way, is one of the 
latest American forms, from the Upper Greensand or Marl of New 
Jersey, and occurring also, if Merriam’s determination is correct, 
in the Niobrara of Kansas. 
The common aquatic ancestor of the three types must be sought 
for in a much earlier period, certainly in the Lower Cretaceous. 
ee 
