nnDLifKA] SKELETAL REMAINS OF EARLY MAN 349 



As to other parts of the ])onc, "the medium portion of the pos- 

 terior arch of the Tetraprothomo [p. 191] is much more massive than 

 in man, more broad or extended from above downward, with its 

 posterior surface very convex and rough, but with a distinctly 

 foraied tubercle. The superior border forms in the median part a 

 curve which is convex upward and the inferior border shows another 

 curve, convex downward, in a manner even more accentuated. In 

 the human atlas the inferior border forms usually a curve concave 

 upward, although there are cases in which the border is straight or 

 plane, but the characteristic convex form of the Tetraprothomo is so 

 rare that among the numerous human atlases that have passed 

 through my hands I have not seen it up to the present, except in a 

 single example, and even there not to a similar degree. It is clear, 

 therefore, that it is a case of atavic reappearance of a characteristic 

 which reaches back to the ancient Microhiotheriidse and which has 

 been retained in the diminutive living dideJfids which constitute the 

 genus Peramys. 



"In the Tetraprothonto the massive form of the median part of 

 the posterior arch, the great breadth of the same in tliis region, and 

 the great consequent extension of the rough surface which gives 

 insertion to the cervical muscles, indicate that the latter were con- 

 siderably stronger than they are in man, and that, therefore, the 

 skull with relation to the stature was of notably larger size. In 

 fact, this region of the posterior arch of the atlas of the Tetraprothomo 

 is so different from that of Homo that it is permissible to suppose it 

 subserved functions which have since become unnecessary." 



This lower portion of the posterior arch leads Ameghino to the 

 conclusion that the Tetraprothomo possessed a special cervical liga- 

 ment of which no traces are encountered in the human atlas, except 

 through atavistic reappearance. This ligament does not indicate 

 that the skull of this being was inclined farther forward than in man ; 

 the Ugament was necessary to insure the erect position of the skull 

 and it could not disappear immediately after tliis was accomplished. 

 "The great development of the cervical hgament in the Tetraprothomo 

 proves that this is really the true forerunner of man ; it demonstrates 

 also that the erect position was in the Tetraprothomo an acquisition 

 so recent that the cervical ligament referred to has not as yet, or 

 has only recently, entered on the road to retrogressive evolution" 

 (p. 194). 



The lateral masses of the atlas with the articulation are, con- 

 sidered as a whole, "essentially human, but with notable deviations 

 wliich bring the generic difference of tlie bone considerably into 

 evidence." 



The transverse processes, now defective, were very small. The 

 superior articular facets are very different from those of the atlas 

 of man; they are but slightly concave antero-posteriorly and the 



