[51 J THE EVOLUTION OF THE FINS OF FISHES. 1031 



views given by Eichardson also show that "0. ioop.s'" is very thick in 

 proportion to its length and height, much more so in fact than it is in 

 either the " Molacanthus^^ or Mola stages of development; a fact which 

 would also favor the conclusion that this represents a much younger 

 stage, and therefore a smaller fish than the two latter. The two upper 

 rows of spines on "0. boops^'' can be pretty closely homologized with 

 those in the same relative position in the Molacantlma stage, but the 

 three median ventral spines present along the median ventral aspect 

 of both the Molacanthus stage and the very young 31ola are absent in 



This close agreement in the distribution of the spines cannot be con- 

 sidered otherwise than indicative of the very close ontogenetic rela- 

 tionship of the three forms in question, and such a conclusion is still 

 further strengthened by the fact that in all three the spines are appar- 

 ently of cuticular origin. They are characteristic and persistent until 

 the young 3Iola reaches a length of about two inches, when they drop 

 off, leaving the skin nearly smooth, as in the adult, though the scars 

 with raised borders where the spines were originally attached remain 

 and occupy the same relative i)osition in respect to each other as in 

 still younger stages and that phase represented by Molacanthus. 



The interval between the dorsal and anal fin of '' 0. boops " shows that 

 even in this early condition of growth the Molina have no true caudal 

 fin, or, at least, a caudal developed with intermediary neural and hse- 

 mal supports, such as are found in normal Teleosts. Yet it is clear that 

 in the Molacanthus stage the first signs of what represents a caudal in 

 Mala appear as very short rays included by a narrow fold of epiblast or 

 skin, which grows in height as the rays lengthen, and approach the con- 

 dition which they present in the young of Mola. But the development 

 of the median caudal rays to a length exceeding twice that of the rays 

 above and below them in young specimens of Mola shortly after they 

 have passed the Molacanthus stage is very remarkable, since, as the fish 

 increases in size, this tail-like extension of the middle rays of the caudal 

 disappears, so that the margin of the whole tail-fin assumes a moder- 

 ately convex or gently rounded outline when viewed from the side. 

 This prolongation of the central rays of the caudal in the young Mola, 

 however, assumes a striking significance when compared with another 

 closely related adult form pertaining to the Molina first described and 

 figured by Bleeker* and now known as Masturus oxyioropterus (Blkr.) Gill, 

 which has the central rays of the caudal very much prolonged and 

 slightly divergent, as in the young of Mola, the backwardly projecting 

 central part of the tail being also armed with a pair of dermal plates. 

 This condition of parallelism in the development of the central rays of 

 the tail of the young o? Mola and of the adult Masturus would indicate 

 that the latter has retained in the course of its evolution a characteristic 

 which is only a transient feature of the evolution of Mola. 



*Med. d. Kon. Akad. Weteuschappen, 2de Reeks, Deel, VII, Amsterdam, 1873. 



