136 HAY. [VOL. I. 



than 56 mm. in length. At this stage the vertebral column is 

 far advanced in its development. 



From the fact that one of the arch-bearing discs of the 

 middle of the tail had united with the archless disc behind it, 

 the authors came to the conclusion that in all parts of the ver- 

 tebral column the archless discs form the posterior halves of 

 the definitive vertebrae. However, in the unions occasionally 

 formed between the arch-bearing and the archless discs of the 

 tail the one without arches is probably more commonly in front. 

 Besides this, the text of my former paper and its Fig. 10 give 

 proof that the elements of the archless disc belong to the an- 

 terior portion of the centrum. This conclusion is confirmed 

 by many other preparations in my possession. Hence, the 

 terms "precentrum" and "postcentrum," employed by Gadow 

 and Abbott, are to be understood in just the opposite sense. 

 If, however, as appears probable, the archless discs in the 

 Stegocephali unite with the arch-bearing disc in front, it would 

 be better to employ terms applicable to both groups. I have 

 been obliged to propose the rather inconvenient terms pleuro- 

 hsemacentrum and epi-hypocentrum. 



Gadow and Abbott also claim to have found, in the trunk 

 region, the lower intercalated cartilages. While admitting the 

 possibility of the correctness of their identification, I am in- 

 clined to believe that they are mistaken. What they regard 

 as these cartilages have, in my former paper, been desig- 

 nated as " aortal supports," and appear to be merely extensions 

 downward from the bases of the lower arches. They are, 

 wherever I have seen them, continuous with these arches, 

 except in the hinder dorsal region, where they seem to be cut 

 off from connection with the down-growth of the lower arches 

 by the rapid development of the bone. Somewhat similar 

 developments of the lower cartilages are found in Acipenser 

 for the protection of the aorta, and in this fish the intercalated 

 cartilages also occur. These so-called interventrals do not 

 occur in the anterior portion of the tail of Amia, where we 

 might expect to find them, in case they were such. 



It is affirmed by the authors that the upper and lower arches, 

 basidorsals and basiventrals, of the tail of Amia do not lie in 



