Williston.] Mosasaurs. 175 



by Merriam. The following specific differences are given by 

 Merriam : 



(1) The mandible, which is not truncated at the tip, but rounded and narrow. 

 (2) The basioccipital, whose hypapophyses are not so strongly compressed, and, 

 parallel to the longitudinal axis of the head, are scarcely half as broad as in T. 

 proriger and T. micromus. (3) The pterygoids send out the transverse process 

 in the vicinity of the sixth tooth. (4) The maxillary teeth, which are rounder, 

 with almost no facets on the outer side or striations, while the inner side shows 

 a strong striation. 



(1) In two pairs of mandibles of each species I can distin- 

 guish no differences in the front end. (2) I can distinguish no 

 differences in the shape of the hypapophyses that are not due to 

 post-mortem origin. (3) In the pterygoids of T. dyspelor the 

 two bones of one skull send off the transverse process opposite 

 the seventh and eighth teeth, precisely where they are in T. 

 •proriger. (4) The teeth of T. dyspelor seem stouter, and possi- 

 bly the characters given may be correct, but I am in doubt. I 

 cannot find characters about which I feel assured. Neverthe- 

 less, there can be no doubt but what the two species are distinct. 

 At present, however, this distinction must rest chiefly upon the 

 very much greater size of T. dyspelor, rather than upon struc- 

 tural characters. Here, as so commonly elsewhere, the specific 

 characters have been generally obliterated by the compression 

 and distortion of the bones. The skull of T. dyspelor measures 

 forty inches in length from the tip of the rostrum to the con- 

 dyle, and the mandible is forty-eight inches long. The quad- 

 rate has a length of seven inches. 



Tylosaurus micromus. 



Hhinosaurus micromus Marsh, Amer. Journ. Sci., June, 1872, pi. xni, 

 ff . 1, 2. 



Liodon micromus Cope, Cret. Vert., 271. 



T ylosaurus micromus Merriam, Ueber die Pyth., etc., 24, pi. i, f. 3. 



Scarcely any of the original characters given by the author 

 of this species are valid. In T. proriger, " which is three or four 

 times larger than the present species, the cervical vertebrae 

 have vertically ovate articular faces," while in T. micromus 

 " the cervical vertebrae have the articular faces but slightly 



