Williston.] Mosasaurs. 143 



in the sediment. The caudal vertebrae are distinctly vertical 

 oval. The spines and chevrons are stouter and shorter in pro- 

 portion to the centra than in Platecarpus. The former, also, 

 show a marked degree of irregularity in their obliquity, as will 

 be seen in the restoration of this species. Probably this irregu- 

 larity is not constant in all the different individuals of the same 

 species. The number of the pygals is somewhat indefinite. 

 In a specimen in the museum in which the vertebral column 

 is complete there are five typical ones and two more with rudi- 

 mentary tubercles for the chevrons. In others these tubercles 

 are somewhat larger. The number is not less than five, and 

 may be seven. * 



Below is given a comparison of the different regions of the 

 vertebral column in the three typical species that have been dis- 

 cussed in this work. 



Number of vertebrce. 



Clidastes. Platecarpus. Tylosaurus. 



Cervical 7 7 7 



Dorsals 35 22 23 



Pygals 7 5 6 



Chevron caudals 70 80? 80 



118 115? 116 



Lengths of the different regions. 



Clidastes. Platecarpus. Tylosaurus. 



Skull 0.420 0.512 0.816 



Neck 226 240 360 



Trunk 1.360 1.345 2.000 



Tail 1.460 2.160? 3.165 



3.466 4.257 6.341 



Proportionate lengths of the different regions of the body. 



Clidastes. Platecarpus. Tylosaurus. 



Skull 12.1 12.0 13.0 



Xeck 6.5 5.6 5.6 



Trunk 39.2..... 31.3 31.5 



Tail 42.2 50.7? 49.9 



100.0 100.0 100.0 



Extremities. 



In all the known Mosasaurs there are four functional limbs, 

 varying not a little in the different genera in size and structure. 

 The arm and leg bones are short and broad, the articular sur- 



