428 



MEMOIRS OF THE CARNEGIE MUSEUM. 



corresponds very well to the type. All these vertebrae, except the atlas, are more 

 or less mutilated, but enough is preserved to show that they are very similar to 

 those bones in D. cooki, size excepted. In its ]iroportions the atlas referred to is 

 not unlike that of the smaller form [D. cooki), save that the transverse process 

 is less extended forward and is somewhat heavier, especially along the terminal 

 border. A second marked difference of this bone in the two species is the presence 



Fig. 1.5. Diccralheriwm niohrarense Peterson, No. 1910, Coll. Carnegie Museum. Anterior and ventral 



views of atlas. X i. 



in D. niohrarense of a round venal foramen, (remnant of the inferior exit of the 

 vertebraterial canal) on the ventral face at the base of the transverse process, and 

 a strong antcro-posteriorly directed ridge immediately internal to the foramen. 

 While there is in D. cooki a groove and occasionally a minute foramen, located 

 in the same position as the foramen described on the atlas of D. niohrarense, there 

 is found no evidence of the ridge in D. cooki. If the heavy terminal border of the 

 transverse process and the venal canal are constant in D. niohrarense, this may be 

 regarded as an additional specific character. (See Fig. 15.) The third cervical 



