WILT.ISTON: NEW OR I.IITLE KNOWN EXTINCT VERTEP.RATES. 1 69 



SO proven by characters hitherto given. I believe that we yet have 

 not a little to learn of these animals from the later American Cre- 

 taceous. In the University Museum there is a large part of an 

 individual from nearly the same horizon as that of M. hofridiis, which 

 I am not yet able to refer to any known genus, and which I believe 

 will prove to be distinct. It is remarkable for the very broad head, 

 short jaws, with only eleven teeth in the maxilla and thirteen in the 

 mandible, stout, unfaceted teeth and peculiarities of the limb bones 

 which distinguish it from Platecarpiis, its nearest ally. The length 

 of the jaws is 1050 mm, and the width of the frontal bone 2S5 mm. 

 while the transverse diameter of the centrum of the third cervical 

 vertebra is 48 mm., showing a remarkably large head for the size of 

 the body. The quadrate is peculiar in having the supracolumellar 

 process co-ossified with the body at its extremity. The species, 

 which I shall call Ovcrloni, for my assistant Mr. T. R. Overton, who 

 discovered it near the top of the Ft. Pierre of the Cheyenne River, 

 will be described and figured at an early date. 



Of these genera I would separate Mosasaiinis (and Plcrycollasaunis 

 if distinct) and Clidasies as a distinct family upon the following 

 skeletal characters. The characters of the skull I will discuss at 

 length in a future paper. 



Mosasauridse Conybeare, lS-34 (C/<'^r.<?//r/'r Copi', Edentosauvidd' 'Marsli). 



Humerus with a prominent radial process at the distal end; carpus 

 and tarsus composed of fully ossified bones closely articulating with 

 each other and with the adjacent bones; hind feet tetradactyl; tail 

 vertically flattened, the trunk relatively long; chevrons co-ossified; 

 sternum calcified. 

 Tylosauridae Marsh (nomeii lunluni), Anipr. .lourn. Sci. xii, .luly, 1870. 



Humerus expanded distally, but without radial or nlnar processes; 

 carpus and tarsus largely cartilaginous, with three or four nodular 

 ossifications not closely articulating with each other or with the 

 adjacent bones; hind feet pentadactyl (?); chevron bones free; tail 

 not vertically flattened and relatively long; sternum and costal 

 cartilages not calcified. 



Dollo suggests that Marsh was in error in ascribing five toes to the 

 hind foot of Platecarpus, but I believe that Marsh was right. The 

 fifth metatarsal in this genus, as also in Tylosaurus, is more elongated 

 than in Cl/dasffs, and has a distal articular expansion. I shall figure 

 shortly the hind foot of a specimen in the museum belonging to 

 Tylosaurus which I think will prove the existence of pentadactylism. 



The discovery of these additional characters of Mosasaurus leaves 

 the generic differences between it and Clidastcs not as conspicuous 



