120 PROCEEDINGS OF THE AMERICAN ACADEMY 



also find in otner genera (Ctenolabrus), a large pigment spot, remain- 

 ing more prominent than the others during the whole embryonic 

 growth, and which can still be traced when the young fish has grown to 

 a considerable size. In Ctenolabrus, the large pigment spot is placed 

 on each side at the base of the tail, half-way between the termination 

 of the dorsal and anal fins. 



In Plate I. fig. 6, the caudal extremity of the chorda is still moi'e 

 arched upwards, the permanent caudal fin projects as far as the ex- 

 tremity of the embryonic caudal, the lower anterior edge is also sepa- 

 rated by a slight indentation from the general line of the primitive 

 embryonic fin-fold, and four or five of the principal caudal rays show a 

 single articulation. 



In this and in the subsequent stage (PI. I. fig. 7), we see the first 

 trace of the gradual disappearance of the embryonic caudal. In PI. 

 I. fig. 7, the permanent caudal projects beyond the embryonic tail, 

 and there are traces of two articulations in a couple of the principal 

 fin-rays. The permanent caudal has also gradually been thrown more 

 upwards (PL I. fig. 8) ; the fin rays becoming more and more parallel 

 with the axis of the body, until they gradually spread, fan-shaped, on 

 each side (after passing through stage PI. I." fig. 9) of a central line, 

 as in PI. I. figs. 10 and 11. 



lu PI. I. fig. 8, we have the first sign of the disappearance of the 

 extremity of the notochord, preparatory to the formation of the uro- 

 style. (See PI. I. figs. 10, 11, and 12.) The permanent caudal is now 

 pointed, projecting far beyond the embryonic caudal, which, in the sub- 

 sequent stage (PI. I. fig. 9), is reduced to a slight lobe ; it becomes 

 still smaller in the next stage (PI. I. fig. 10) ; and is finally reduced to 

 a mere thickened semi-transparent edge, — the last remnant of the origi- 

 nal embryonic fin-tail fold to be found in the permanent tail (PI, I. 

 fig. 11). Accompanying the disappearance of the embryonic caudal, 

 we find a constant increase in the length of the permanent caudal (PI. 

 I. fig. 9). From being pointed, as in PI. I. figs. 8, 9, it becomes some- 

 what rounded ; and, with the more symmetrical arrangement of the 

 fin-rays, the edge becomes scalloped as in figs. 10 and 11, and it 

 does not differ materially from that of the adult in its general outline. 

 In fig. 11, we can plainly see the ossification of the vertebrae, with the 

 corresponding apophysis, the urostyle, the two principal cartilages sup- 

 porting the fin-rays, with the dorsal and ventral cartilages supporting 

 the accessory fin-rays. This is better shown in fig. 12, — a magnified 

 drawing of the base of the tail of fig. 11. 



The other genera of bony fishes in which I have traced the pres- 



