164 



ON THE EXTINCT MAMMALIA OF 



besides the other unprotruded permanent premolars, together with the protruded 

 permanent true molars, exhibits a form which is a less developed and reduced one of 

 the other premolars. 



Another fossil specimen, consisting of a fragment of the right side of a lower jaw, 

 contains the last premolar and the true molars very much worn. The premolar in 

 its much worn condition has assumed much the apjiearance of that of the second pre- 

 molar of the Sheep in a similar stage of abrasion. The enamel ^Jits are obliterated 

 on the triturating surface of the first molar, and ajjpear as minute oval rings on the 

 second molar. The last molar preserves its anterior two pits, and is inserted into the 

 jaw by fangs. 



Several other, but less perfect fragments of lower jaws, and a few isolated inferior 

 molar teeth, exhibit the same characters as those already given. 



There were no fragments of upper jaws or teeth in the Niobrara or other 

 collections of fossils, which could with any probability be referred to Merycodus 

 necatus. 



Two astragali and a metacarpal bone of some small ruminant, in the Niobrara 

 collection of fossils, may belong to Merycodus necatus. 



One astragalus measures three-fourths of an inch in length and five lines in 

 breadth. The other specimen, somewhat imperfect, is rather larger than the former. 



The metacarpal bone measures four inches and a half in length, and is seven and 

 a half lines wide at the articular extremities. 



The measurements of the lower jaw and inferior molar teeth of Merycodus necatus 

 are as follow: 



Lines. 



6 



61 

 25 

 16 



Depth of lower jaw at first premolar, .... 



Depth of do. at middle of first true molar, .... 



Length of series of six molar teeth, .... 



Length of series of true molars, ..... 



Length of crown of the less than half protruded and moderately worn last 



true molar, ........ 8 



Antero-posterior diameter of do. at middle, . . . . . 7 J 



Antero-posterior diameter of second true molar, . . . .5 



" " first true molar, . . ... 4 



" " last premolar, . . . , . 3J 



" '* second premolar, . . . .3 



" " first premolar, . . . . 2i 



Length of hiatus from first premolar to back border of the mental foramen, 10 



Depth of jaw below hiatus where narrowest, . . . .4 



The collection of fossils obtained by Dr. Hayden in his trip to Dakota in 1866 



