[53] THE EVOLUTION OF THE FINS OF FISHES. 1033 



fore, is that the larval axis was relatively much longer than that of tlie 

 adult, and that it possibly had a lophocercal tail, or at least a caudal 

 prolongation which was probably absorbed at an early period, some- 

 what after the manner of the tail of the taillens Batrachians, and for 

 this reason the end of the primitive caudal axis has not become the 

 support for the caudal fin as in other Teleostei. The development of 

 a special mode of support for the caudal has consequently occurred, 

 which has apparently been caused by the abortion of the posterior end 

 of the primitive axial skeleton. This view of the facts is demonstrated 

 beyond any doubt whatever when wo find that the interneural and 

 interhaemal pieces which support the caudal of Mola are serially ho- 

 mologous with the interspinous pieces which support the dorsal and 

 anal fins, though the interspinous elements of the caudal are shoved 

 out of position and are arranged in two confluent series above and 

 below the osseous axis at right angles to the rows supporting the 

 dorsal and anal. In Fig. 7, Plate VIII, I have attempted to show the 

 caudal of Mola somewhat as it would appear if developed normallj'; the 

 elements which now exist are represented in black, while those which 

 have been suppressed are represented by shaded outlines. The exten- 

 sion of the chordal axis posteriorly beyond the "floating" vertebra is 

 shown in dotted outlines, while the suppressed myotomes and skeleto- 

 genous material of the median vertical portion of the urosome is limited 

 in front approximately by the two heavy, curved, dotted lines, with the 

 arrows alongside. Four more interhsemal elements are shown to be de- 

 veloped, in the young, below the line of the vertebral axis than above it. 



This diagram represents the structural condition with which we have 

 to deal in clearing up the question of the ori^n of the tail of Mola. If 

 the eleven inferior and the seven superior interhaemal pieces are now 

 swung forward in the direction of the arrows and along the course of 

 the heavy dotted curved lines, the interhsemal pieces will have their i^rox- 

 imal ends brought against the posterior face of the neural and haemal 

 spine of the sixteenth vertebra. This must have been the case, for we 

 have shown that the posterior portion of the axial skeleton has been 

 suppressed, but the superior and inferior margins of the urosome have 

 evidently been preserved so that the interhsemal and interneural pieces 

 belonging thereto have developed, but no remains of the corresponding 

 neural and haemeal arches are to be found, so that we have the anoma- 

 lous condition of the interneural and interhsemal supports of the caudal 

 resting upon the neural and haemal arches of a vertebra in advance of 

 those which would have been developed with their corresponding homo- 

 dynamous spines, had the posterior and axial part of the tail not been 

 suppressed. 



The haemal and neural supports corresponding to the seven superior 

 and the eleven inferior interspinous bones have failed to develop, ap- 

 parently, because the skeletogenous tract from which they are normally 

 developed was suppressed, together with the posterior part of the caudal 



