STRUCTURE AND DEVELOPMENT OF LEl'IDOSTKUS. 803 



that it is moreover convenient to retain the term homocercal for 

 those types of caudal fin in which the dorsal lobe has atrophied 

 so far as not to project beyond the ventral lobe. 



We have stated these now well-known facts to enable the 

 reader to follow us in dealing with the comparison between the 

 skeleton supporting the fin-rays of the ventral lobe of the caudal 

 fin, and that supporting the fin-rays of the remaining unpaired 

 fins. 



It has been shewn that in Lepidosteus the unpaired fins fall 

 into two categories, according to the nature of the skeletal parts 

 supporting them. The fin-rays of the true ventral lobe of the 

 caudal fin are supported by the spinous processes of certain of 

 the haemal arches. The remaining unpaired fins, including the 

 anal fin, are supported by the so-called interspinous bones, 

 which are developed independently of the vertebral column arid 

 its arches. 



The question which first presents itself is, how far does this 

 distinction hold good for other Fishes ? This question, though 

 interesting, does not appear to have been greatly discussed by 

 anatomists. Not unfrequently the skeletal supports of the 

 ventral lobe of the caudal fin are assumed to be the same as 

 those of the other fins. 



Davidofif 1 , for instance, in speaking of the unpaired fins of 

 Elasmobranch embryos, says (p. 514): "The cartilaginous rays 

 of the dorsal fins agreed not only in number with the spinous 

 processes (as indeed is also found in the caudal fin of the full- 

 grown Dog-fish)," &c. 



Thacker 2 , again, in his memoir on the Median and Paired 

 Fins, states at p. 284 : " We shall here consider the skeleton of 

 the dorsal and anal fins alone. That of the caudal fin has 

 undergone peculiar modifications by the union of fin-rays with 

 luemal spines." 



Mivart 3 goes into the question more fully. He points out 

 (p. 471) that there is an essential difference between the dorsal 

 and ventral parts of the caudal fin in Elasmobranchs, in that in 



1 " Beitrage z. vergl. Anat. d. hinteren Gliedmassen d. Fische," Morph. Jahrbuch, 

 Vol. v., 1879. 



* J^rans. of the Connecticut Accui., Vol. III., 1877. 



:l St George Mivart, "Fins of Elasmobranchs," Zool. Trans., Vol. X. 



