2li 



fii'eiiale) wliioh is considerably larger on the left side than on the 



Fig. 4. 

 Atlas with foramina arfUF.iia metlialia. 



right one, and at last Fig. 5 gives iis the representation of an atlas, 

 wliicli is already remarkable on account of the existence of a stronglj' 

 developeii bi-iateral ponticulus posterior, but winch shows moreo\er 

 an extraordinary deep de|)ression (impressio niediana areas posterioris) 

 lying in the centre of the areas posterior, a piercing oftlie posterior 

 arch as in the specimens represented in fig. 3 and 4 is however not 

 found here. 



Fig. 5. 

 Atlas with impressio mediana arcus posterioris. 



In the occurrence of these variations, to which till now but little 

 attention lias been paid, I suppose, I nuiy see a proof for the view 

 described above and a priori probable, that also the central part of 

 the arcus posterior atlantis contains elements that must be reduced 

 to the above-mentioned semi-segment la. In that case the notch 

 running transvcrsally, and the foramina arcualia medialia or mediana, 

 eventually occurring in it, would indicate the bonndary-line lietween 

 semi-segment la and senn-segment V>. 



If this sup[)Osition agrees with the actual fact, it follows from 

 what has l)een said, that also in case the ponticuli posteriores and 

 laterales have not develo|)ed, as is most frequently the case with 

 man, the atlas cannot be called equivalent to the other vertebrae, 

 but that also in normal circumstances it has l»een built of elements 

 belonging to 3 semi-segmenls. 



I have projected Fig. 6 (p. 212) in oi-der to give a concise survey 

 of the manner in which 1 conceive the part that the semi-segment 

 hi has in the construction of the atlas with the variations described 



