126 PROCEEDINGS OF THE AMERICAN ACADEMY 



DESCRIPTION OF THE PLATES. 



LETTEnmo. 



a. Anterior cartilage on lower side of notochord, supporting principal fin- 

 rays. 



h. Posterior cartilage on lower side of iiotochord, supporting principal fin- 

 rays. 



a'. Anterior dorsal cartilage. 



y. Posterior dorsal cartilage, supporting accessory fin-rays. 



c. Embryonic caudal fin. 



dd. Principal caudal rays. 



d'. Accessory dorsal caudal rays, above notochord. 



d". Accessory ventral caudal rays, below notochord. 



df. Dorsal fin. 



vf". Ventral fin. 



f. Permanent caudal fin. 



n. Notochord. 



V. Last ossified vertebra. 



v'. Dorsal apophysis. 



v". Ventral apophysis. 



w. Urostyle. 



PLATE L 



Tail of Flounder. 



g. 1. Tail of young fish, with straight notochord and embryonic fin. 



2. Slightly older — the extremity of notochord somewhat arched, and 

 showing first trace of caudal fin. 



3. The indentation between the embryonic caudal and the permanent 

 caudal is deeper; fin-rays well defined. 



4. Extremity of notochord still more arched than in preceding figures ; 

 the separation between the permanent and embryonic caudals 

 somewhat more distinct. 



, 5. In this stage, the permanent and embryonic caudals form a sharp an- 

 gle ; the distinction between embryonic and permanent rays is 

 well shown. 



, 6. The permanent caudal extends as far as the embryonic caudal, which 

 now shows traces of resorption. 



, 7. The pointed permanent caudal extends beyond the line of the em- 

 bryonic caudal, somewhat decreased in size. 



, 8. The cartilaginous supports of the fin-rays proper have become large ; 

 the extremity of the notochord shows traces of the formation of 

 the urostyle. 



, 9. The permanent caudal has increased greatly in length ; the embry- 

 onic caudal is now reduced to a small rounded lobe. 



, 10. The caudal has become well rounded ; a mere trace of the embryonic 

 caudal is left. The urostyle is also more distinct. 



