118 



PROCEEDINGS OF THE AMERICAN ACADEMY 



the caudal fin, capable of independent vibratory motion ; and compared 

 the young caudal to a second anal, from its position as an appenda'ge 

 of the lower surface of the dorsal column. 



This filament still exists in specimens having a length of eight 

 inches. (See fig. 1.) Professor Wilder has lately * followed the trans- 

 formations of the tail of the Gar-pike, from the time when the tail of 

 the young Lepidosteus is in the stage corresponding to that of PL I. 

 fig. 4, of this paper, until the filament has entirely disappeared, and the 

 tail has assumed the rounded outline of the adult. He has also found 

 traces of this filament in very young specimens of Amia, as a slight 

 undulation of the dorsal edge of the caudal at the termination of the 

 supposed notochord. 



The stage figured by Wilder agrees very closely to the stage of PI. 

 I. fig. 10, of this note, and represents — I have not the least doubt, from 

 having traced its gradual disappearance in so many of our bony fishes 

 — the remnant of the fleshy embryonic filament of our bony fishes 

 and of the Ganoids, as was surmised by Wilder. 



I have given in PI. I., quite in detail, the changes gradually taking 

 place in the tail of a Flounder, from the time it leaves the egg until it 

 has nearly assumed the final shape of the adult. On Plate II., I have 

 given figures of a number of young fish tails, of different species, to 

 show how general is the presence of the embryonic caudal fin, even in 

 a comparatively advanced stage of growth. I have also given on the 

 same Plate a figure of a young Lophius (PI. II. fig. 9), a few days 

 after its hatching from the egg, to show how extensive are the changes 

 our fishes go through before reaching the adult condition ; and I hope 

 to give little by little, in papers I am now preparing, the general his- 

 tory of these changes in the principal families of our marine fishes, 

 commencing with the development of the Pleuronectidae. 



In a young Pleuronectes, just hatched from the egg (PI. I. fig. 1), 

 the caudal end of the chorda is straight. It extends from the anterior 

 arch between the otolites to its posterior extremity, in a line nearly 

 parallel to the dorsal embryonic fin, nearer the dorsal than the ventral 

 side. The embryonic caudal fin is rounded, and nearly symmetrical 

 above and below, the dorsal fold being the narrowest. 



In the next stage figured (PL I. fig. 2), the caudal extremity of the 

 chorda has become slightly bent upwards, concave towards the ventral 

 side ; and then appears the first trace of the division line f be- 



* Notes on the American Ganoids. Proc. Am. Ass. Adv. Sc, 1876, p. 153, 

 Detroit Meeting. 



