40 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



stage, from the two lower intercalated cartilages of Amia. It seems 

 to me, therefore, that we must recognize these two lower intercalated 

 cartilages and their ossifications as representing the hypocentrum pleu- 

 rale. For the sake of brevity I propose in this paper to employ for this 

 bone the name hcemacentrum, although the term is not wholly satisfac- 

 tory. The vertebra which has hitherto been called the pleurocen- 

 trum, is therefore a pleuro-haemacentrum. 



In the Aimioid fishes, and in the stegocephalous Amphibians, as 

 has been described, the lower surface of the notochord is often em- 

 braced, in each segment, by a curved plate of bone which rises on 

 the sides of the notochord and to which the lower arch is closely 

 attached. This plate, sometimes divided into two lateral halves, has 

 received the name hypocentrum. In Amia, the ossifications which 

 produce the lower portion of each centrum that is provided with 

 upper and lower arches, spread from the bases of the lower half-arches 

 of that vertebra. These ossifications belong to the arches from which 

 they spring as truly as the ossifications connected with the interca- 

 lated cartilages belong to the latter. As already stated they meet in 

 the middle line below and grow upward on the sides of the notochord. 

 These horse-shoe-shaped pieces of bone must therefore correspond 

 to the hypocentra of the ancient representatives of Amia, and doubt- 

 less to the hypocentra of the Stegocephali. I would therefore define 

 the hypocentrum as the vertebral element which results from the 

 union of the ossifications arising from the bases of the lower arches. 

 To the hypocentrum may be attached by suture or coossification the 

 bones springing from the remainder of the lower arch. In the trunk 

 region of Amia the parapophyses must be regarded as portions of the 

 hypocentrum ; In the tail region, the lower arches, which enclose the 

 blood-vessels, are morphologically distinct bones, although they be- 

 long to the same cartilaginous arches as the hypocentra. 



In Amia the hypocentra of the middle tail region grows upward 

 on the sides of the notochord and unite with the ossifications which 

 grow downward from the corresponding upper arches. The verte- 

 bral centra thus formed are those which have borne the name of 

 hypocentra, or intercentra. They are not hypocentra simply. We 

 might call the bone resulting from the union of the ossifications of 

 the bases of the upper arches, epicentra. The vertebrae now under 

 discussion would then be epi-hypo-centra. 



In the dorsal and anterior tail regions the vertebrae are produced 

 each by the union of the hypocentrum and the pleurocentrum. They 

 are therefore pleuro-hypocentra. 



