OF ARTS AND SCIENCES. 119 



tween the embryonic and the permanent caudal fins ; also, traces of the 

 principal caudal rays, and of the accessory rays both of the dorsal and 

 ventral side of the tail. In the following stage, the indentation be- 

 tween the embryonic and the permanent caudal has become deeper, 

 the chorda more arched ; the caudal fin-rays are well marked, and the 

 permanent caudal now projects well beyond the general outline of 

 the embryonic fin fold. The delicate lines imitating the fin-rays of 

 the permanent fins, are specially prominent in subsequent stages (PI. I. 

 figs. 5-9). In the tail of the young of PI. I. fig. 4, the whole tail is 

 thrown up, and has now assumed the regular heterocercal type ; the 

 permanent fin-rays of the caudal being all placed on the lower side of 

 the chorda. There is no trace, as yet, of any ossification of the verte- 

 bral column ; the anterior and posterior supports of the fin-rays, as 

 well as the supports of the accessory dorsal and ventral rays, are all 

 cartilaginous. 



In the next stages (PI. I. figs. 5-7), the embryonic caudal has 

 assumed the shape of a large independent lobe ; the permanent fin 

 proper extending entirely below it, and forming an independent fin, 

 like a second anal, entirely on the lower surface of the notochord. The 

 resemblance of the tail, at this stage, to the tail of Lepidosteus is so 

 striking that I have here given, for comparison, a figure of the tail of 

 the young Lepidosteus (eight inches in length), from which the late 

 Professor Agassiz described the fleshy filament extending indepen- 

 dently above the permanent caudal. 



Fig. 1. Tail of young Lepidosteus. 



In consequence of the greater arching of the notochord (PI. I. fig. 5), 

 and the simultaneous growth of the permanent caudal (the embryonic 

 caudal remaining unchanged), the caudal fin is now bilobed. The prin- 

 cipal permanent fin-rays of the tail-fin are well developed, but not yet 

 articulated ; the dorsal and ventral cartilages of the accessory fin-rays 

 are well separated. In this and the preceding stages, large pigment 

 spots are found between the two principal cartilaginous supports of 

 the fin-rays, and afterwards greatly developed along their outer edge. 

 The large black spot found on the tail of Amia, near the base of the 

 caudal rays, recalls strikingly this space covered by pigment spots. We 



