802 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



verted to an embryonic condition : an evolutional process which 

 is of more frequent occurrence than has usually been admitted. 



In the caudal region there is almost always developed in the 

 larvae of the above groups a special ventral lobe of the em- 

 bryonic fin a short distance from the end of the tail. In Elasmo- 

 branchii and Chondrostean Ganoids the portion of the em- 

 bryonic tail behind this lobe persists through life, and a special 

 type of caudal fin, which is usually called heterocercal, is thus 

 produced. This type of caudal fin appears to have been the 

 most usual in the earlier geological periods. 



Simultaneously with the formation of the ventral lobe of the 

 heterocercal caudal fin, the notochord with the vertebral tissues 

 surrounding it, becomes bent somewhat dorsalwards, and thus 

 the primitive caudal fin forms a dorsally directed lobe of the 

 heterocercal tail. We shall call this part the dorsal lobe of the 

 tail-fin, and the secondarily formed lobe the ventral lobe. 



Lepidosteus and Amia (Wilder, No. 15) amongst the bony 

 Ganoids, and, as has recently been shewn by A. Agassiz 1 , most 

 Teleostei acquire at an early stage of their development hetero- 

 cercal caudal fins, like those of Elasmobranchii and the Chondro- 

 stean Ganoids ; but in the course of their further growth the 

 dorsal lobe partly atrophies, and partly disappears as such, 

 owing to the great prominence acquired by the ventral lobe. A 

 portion of the dorsally flexed notochord and of the cartilage or 

 bone replacing or investing it remains, however, as an indication 

 of the original dorsal lobe, though it does not project backwards 

 beyond the level of the end of the ventral lobe, which in these 

 types forms the terminal caudal fin. 



The true significance of the dorsally flexed portion of the 

 vertebral axis was first clearly stated by Huxley 2 , but as 

 A. Agassiz has fairly pointed out in the paper already quoted, 

 this fact does not in any way militate against the view put 

 forward by L. Agassiz that there is a complete parallelism be- 

 tween the embryonic development of the tail in these Fishes 

 and the palseontological development of this organ. We think 



1 " On the Young Stages of some Osseous Fishes. I. The Development of the 

 Tail," Proc. of the American Academy of Arts and Sciences^ Vol. XIIL, 1877. 



2 "Observations on the Development of some Parts of the Skeleton of Fishes," 

 Quart. Journ. of Micr. Science, Vol. VII., 1859. 



