[47] THE EVOLUTION OF THE EINS OF FISHES. 1027 



dorsal rays have been pushed forward in the young, though the most 

 anterior dorsal rays resting on the head of the adult above the snout 

 may have been developed during post-larval life. This peculiarity of 

 development is approached by the fore part of the dorsal of Flounders, 

 in the young of which, as development proceeds, the anterior dorsal 

 rays are advanced so as to occupy a more anterior position in the adult 

 than in the very young. Such examples of the displacement of struc- 

 tures forwards in the course of development would probably lend sup- 

 port to Gegenbaur's interpretation of the cephalic fin of Tor])edo as a 

 l)art of the pectoral which has been advanced and acquired support 

 from the antorbital processes of the skull secondarily. 



IX.— On the TRANSFORMATIONS OF THE TAIL OF MOLA. 

 (See Plate VIII.) 



In 1870 F. W. Putnam* described the anatomical peculiarities of 

 Molacanthus nummularis (Walb.), G-ill, as compared with the young of 

 Mala rotunda, Cuv. In the latter, Putnam states that he found "the 

 neural spines of the fifth to the fifteenth vertebra closely packed to- 

 gether with the interneural spines and extending back wards to support 

 the dorsal fin, while the haemal spines of the tenth to the sixteenth ver- 

 tebra? are in close connection with the expanded interhsmal spines sup- 

 porting the anal fin. The sixteenth vertebra gives off large neural and 

 hasmal spines, the former having five interneural spines anchvlosed 

 [in contact] with it as in the adult, while the hgemal spine supports [or 

 is in contact with, posteriorly] nine interh^emal spines, the lower one 

 of which belongs to the anal fin, while the others are of the caudal chain. 

 In the adult only seven interhaemal spines are connected with this hge- 

 mal spine. The seventeenth vertebra in the adult lies in the caudal 

 chain of interspinous bones, and from its being separated from the ver- 

 tebral column has been as often considered an interspinous bone as a 

 vertebra. In the young specimens this vertebra, though separated from 

 the column as in the adult, has in close connection with it two bones 

 above and two below, probably indicating that this vertebra is in reality 

 the consolidation of two vertebral bodies, the seventeenth and eighteenth, 

 while two other small (neural and haemal) bones posterior to this free 

 vertebra indicate that a nineteenth vertebra existed at an earlier stage. 

 These six neural and haemal (three each) bones disappear in the adult, 

 and with them the central rays of the caudal fin, and they and the seven- 

 teenth, eighteenth, and nineteenth vertebra? are represented only by 

 the free or ' floating' seventeenth rertebra, which lies in the chain of 

 mterspinons bones of the caudal. This is the only instance of a vertebra 

 existing as distinctly separated from the verteijral column, known to 

 the author." * * ♦ 



" The skeleton of Molacatithus shows the interspinous bones of the dor- 

 * American Naturalist, 1874, IV, 629-633. 



