OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 51 



a large number of the Amniota. As a result of subsequent modifi- 

 cations, the rib-head may, of course, form an attachment with the 

 centrum, usually with the one next behind ; occasionally to the hinder 

 end of the one in front. 



In case the rib develops a second point of attachment to the ver- 

 tebral elements, this will naturally be with the upper arch, since rib 

 and arch lie in the same transverse septum ; the tubercular portion of 

 the rib will also be carried backward, and this may have some in- 

 fluence in causing also the head of the rib to take a more posterior 

 articulation. As a matter of fact, the tubercular process of the ver- 

 tebra usually springs from the upper arch. The Ichthyosauria fur- 

 nish us with a case in which the tubercular attachment has descended 

 low down on the centrum. In the dorsal region of the Crocodilia the 

 rib-heads also desert the centra and adhere to the transverse pro- 

 cesses. 



We have seen that in Amia the pleurocentrum, both in the dor- 

 sal and in the middle portion of the tail, goes to form the anterior 

 portion of the vertebra to which it belongs. On the contrary, Fro- 

 riep holds that the upper arch, rib, and "hypochordale Spange" belong 

 primarily to the centrum immediately behind them, thus making the 

 hypocentrum anterior. The true explanation may be that the hypo- 

 centrum and the elements connected with it belong no more to the 

 pleurocentrum behind it than to the one in front. It is then free to 

 form connection with either, and this connection is different in the 

 different groups of vertebrates. 



In concluding this paper I wish to acknowledge my indebtedness 

 to Dr. George Baur for the opportunity to conduct my investigations 

 in his laboratory in the University of Chicago, and for many val- 

 uable suggestions offered during the progress of my work. 



