16 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



whose bases are seen on the surface of each of the vertebrae, 

 while their apices reach usually to the notochordal sheath. The car- 

 tilage appears to persist even in adult life. It is ensheathed in a thin 

 layer of bone, and in cross sections this is seen to extend outward as 

 far as does the cartilage itself. Hence when the cartilage shrinks 

 through drying, there is left a little rim of bone surrounding the pit. 

 Inmost of the trunk region, the bases of these cartilages do not come 

 into contact with the intravertebral bases of the transverse processes 

 (Fig. i). In the posterior dorsal region, however, where the pro- 

 jesses have descended considerably, the cartilages in question come 

 into contact with the intravertebral bases of the lower arches, 

 and soon have the appearance of being suspended from their lower, 

 or inner, sides (Fig. 5). Toward the last dorsal vertebra these 

 cartilages, which, from their relation to the aorta, may be called the 

 aortal supports, become shorter. In the last dorsal vertebra (Fig. 6) 

 the cartilage is short, and is attached to the intravertebral basis of the 

 haemal arch half-way from the notochord to the outer surface of the 

 vertebra. In the vertebras of the tail the cartilages are missing. 

 There is, however, in my younger specimen, what seems to be ves- 

 tiges of them in the first caudal vertebra. Nothing, however, can be 

 more certain than that the lower arches of the trunk are bent down 

 to form the arches of the tail, and that the aortal supports have 

 nothing to do with the formation of the caudal haemal arches. In the 

 last dorsal vertebra we find, at the anterior end, the aortal supports 

 looking somewhat like the lower arch-bases. Further back, 

 however, the latter come into view, in a series of sections, and 

 the aortal supports cling to their inner surfaces (Fig. 6). Figure 

 7 shows a section through the first caudal. The bases of the lower 

 arches are seen in same position as in the last dorsal. 



The origin and the homology of these aortal supports will be fur- 

 ther considered when we come to examine younger specimens. In 

 many, if not most, of the osseous fishes, there is to be found on the 

 under side of each of the dorsal vertebrae, a pair of bony ridges, one 

 on each side of the aorta. To these ridges the aorta appears to be 

 suspended. In the anterior portion of the vertebral axis of Acipenser 

 a plate of cartilage grows downward from the lower edge of the base 

 of each half of the haemal arch, and gives support and protection to 

 a portion of the aorta. Further back, these cartilaginous plates bend 

 inward towards each other, and finally unite below the aorta, thus 

 enclosing it in a canal. It appears to me that the aortal supports of 

 Amia may be regarded as the equivalents of those cartilaginous pro- 

 cesses which protect the aorta in Acipenser. In Amia the cartilages 



