"Williston.] Mosasaurs. 87 



RANGE AND DISTRIBUTION. 



The Mosasaurs are at present known from four remote re- 

 gions of the word — North and South America, Europe, and 

 New Zealand. Doubtless they lived over the greater part of 

 the earth, and may be expected wherever marine Upper Creta- 

 ceous deposits occur. Their geological range is confined exclu- 

 sively to the Upper Cretaceous, from the time corresponding to 

 the upper part of the Dakota to that of the lower part of the 

 Laramie, or from the Upper Cenomanian to the Lower Danian. 

 The correlation of the American Cretaceous deposits with those 

 of Europe, or even with each other, is by no means exact, or 

 even approximately exact. Nevertheless the equivalency of the 

 different strata and epochs is sufficiently well determined to 

 admit of approximate results. 



The oldest Mosasaurs are apparently those described by 

 Hector from New Zealand, which he referred to the genera 

 Liodon and Taniichasaurus Hector. 35 The genus Liodon Owen, 

 Dollo has recently shown to be a synonym of Mosasaurus. 36 

 Whether or not Hector's species is congeneric with those placed 

 under Liodon by Cope is not certain, though it is evident that 

 it is closely allied. Taniwhasaurus is clearly of the Platecarpus 

 type, and may possibly belong to that genus. 



The most recent form is the historical Mosasaurus giganteus 

 Soemmering (M. camperi, M. hoffmani) , from the Maestricht 

 beds in the Lower Danian. These three forms, Tylosaurus, 

 Platecarpus, and Mosasaurus, represent three distinct and diverg- 

 ent types, which I have called the Tylosaurinae, Platecarpinse, 

 and Mosasaurinee, corresponding to the megarhynchous, micro- 

 rhynchous and mesorhynchous types of Dollo. 37 



The Tylosaurinse begin with Liodon (Tylosaurus f) haumur- 

 iensis Hector in the Cenomanian of New Zealand, and continue 

 to the Upper Senonian of Belgium as found in the genus Llaino- 

 saurus Dollo, from the brown phosphatic chalk of Mesvin 



35. On the Fossil Reptilia of New Zealand, Trans, and Proc. New Zealand Institute, vi, 338, 

 1873. 



36. Bulletin Soc. Belg. Geol., vn, 79, 1892. 



37. Mem. Soc. Belg. de Geol., iv, 163, 1890. 



