271 



quent discoveries of Professor Marsh ; and they appear hardly sufficient to 

 justify the- name of Pythonomorpha. 



The remains of mosasauroid reptiles are comparatively abundant in the 

 Cretaceous formation of the United States. The specimens collected have 

 formed the basis of a multitude of species and genera, the number of which 

 will probably be somewhat reduced on more careful study and comparison of 

 the materials. 



In the description of the few mosasauroid remains which have been sub- 

 mitted to my examination, I have referred them to species for the most part 

 as recently named by Professor Marsh, who, with the rich materials in his 

 possession, has the best opportunity of determining their generic and specific 

 characters. 



TYLOSAURUS. 



Tylosaurus dyspelor. 



Among the fossils submitted to my examination by the Smithsonian Insti- 

 tution, there are some bones of a large mosasauroid animal, collected by Dr. 

 W. B. Lyon, United States Army, in the vicinity of Fort McRae, New Mexico. 

 They consist of vertebrae, mostly more ,or less crushed and otherwise muti- 

 lated, and a few limb-bones, and were obtained from a stratum of soft, yel- 

 lowish chalk. Specimens from the same collection and skeleton were de- 

 scribed by Professor Cope, and referred to a species with the name of Liodon 

 dyspelor. This was subsequently referred to a genus, by Professor Marsh, 

 with the name of Rhinosaurus, which, being pre-occupied, Professor Cope 

 proposed that of Rhamphosaurus, and, as this also was previously appropri- 

 ated, Professor Marsh has now proposed the name of Tylosaurus. 



Of the specimens selected by me for examination half a dozen consist of 

 centra and parts of others of posterior dorsal vertebra?, most of which are 

 remarkable for the extent of compression they have undergone with little 

 appearance of fractures. They look as if they had been in a plastic condi- 

 tion, and in this state had been flattened from above downward. 



In three of the specimens, consisting of posterior halves of dorsal centra, 

 the articular ball presents a half oval outline below, with slanting sides above, 

 and an emarginate summit. The measurements of the ball, indicating a suc- 

 cessive increase in the degree of flattening in the three specimens, arc as 

 follows : 



