[57] THE EVOLUTION OF THE FINS OF FISHES. 1037 



not, as I fear it may be supposed, merely to demolish the grounds upon 

 which the Molacanthidw have been recognized as a family. 



A remarkable reduction in the number of caudal rays occurs in Mola 

 during the period of growth intervening between the tiiue when it is 

 about two inches long and the time when it is full grown. In the young- 

 two inches long there appear to be about twenty-three caudal rays de- 

 veloped ; in the adult, on the other hand, if Wellenbergli's figure is to 

 be relied upon (Fig. 9, PI. VIII) — and it is seemingly very accurate — 

 there are only thirteen caudal rays. This elision or loss of rays seems 

 to occur in the central part of the caudal, while those rays which are 

 distinctly epasial and hypaxial in position seem to be preserved. The 

 tail-like process in the center of the tail-fin of the young, together 

 with its six nearlj^ simple filamentous rays, appears to be entirely ab- 

 sorbed and suppressed. To what extent the filaments represented by 

 Putnam extend beyond the margin of the caudal of the young 1 am un- 

 able to say 5 but of this I am certain, that the epaxial and hj^paxial 

 caudal rays are not simple, as figured by Putnam, but are dichotomous 

 and soft, having three to four terminal branches. The specimens which 

 I have in my possession for study do not show the radial filaments or 

 their divided ends exserted to anything like the extent re})resented by 

 Putnam, and I am very much inclined to believe that the specimens 

 which are before me are normal, and have' never had any such naked 

 projecting filaments. 



It is certain, however, that the dichotomous character of the caudal 

 rays of the young of 3Iola does not persist, but entirely disappears 

 by about the time that they assume their final condition in the adult, 

 and that what with their reduction in number to thirteen and the loss 

 of their dichotomous character and the retreat of their a])ices from 

 the extreme margin of the caudal fin, the fin, as a whole, undergoes 

 even a tertiary metamorphosis in passing from the young form dis- 

 tinctly recognizable as Mola to the adult condition. We thus find that 

 3Iola presents the most extraordinary series of transformations in re- 

 spect to the development of its caudal rays to be found anywhere 

 amongst Tcleost fishes, the origin of which we can explain only upon 

 the ground that the abbreviation of the tail of the larva and the sui^- 

 l)ression of the primitive terminal somites of the body have been more 

 extensive in this instance than in any other. Additional morphological 

 and embryological proof of the position which I have taken Avill hardly 

 be necessary, but it may be well to call attention to the fact that the 

 remarkable arrangement of the muscles of this family of fishes is also 

 unique, yet it is not impossible to explain even this modification ac- 

 cording to the general theorj^ of development, as will be seen in the 

 sequel. When we remember that the vertical fins of Mola are moved 

 by a series of powerful muscles, the substance of which comprises al- 

 most the whole of what corresponds to the lateral muscular masses of 

 normal Teleosts, we must conclude that what is ordinarily developed in 



