103G REPORT OF COMMISSIONER OF FISH AND FISHERIES. [56] 



expect these elements to be rudimentary, or even wanting ; bnt they 

 doubtless primarily develop in cartilage, as in other forms. Their de 

 velopment has evidently been belated or retarded as compared with the 

 other vertical fins. 



The vertebral axis of Molacanthus is apparently very oblique and about 

 parallel with the base of the dorsal, the interval between them being- 

 but two millimeters 5 the distance from this axis to the extreme ventral 

 margin of the abdomen is seven times as great; so that here we have a 

 character strong enough for the most enthusiastic taxonomist to avail 

 himself of as a family character. But let us see if even this is of any 

 value. The fact is tUat tbe axial column of Mola gradually assumes a 

 more ventral i>osition, i. e., the abdominal contour becomes less promi- 

 nent with the advance in age, so that the axial column is apparently 

 shoved downwards, as may be seen upon comparing together figures 

 of the skeletons of the difterentages. This pushing down of the verte- 

 bral column is, however, more apparent than real, the fact being that 

 the abdomen diminishes in height, as a result of which the vertebral 

 axis seems to be j^ushed downwards. During this process the obliquity 

 of the axial column also diminishes, so that its anterior is lowered more 

 than its posterior end. 



During the Molacanthus stage the skeleton of the tail bears every evi- 

 dence of immaturity. It is soft and flexible, and not ossified as flinty 

 bone, as in the adult. The chorda comprises the bulk of the axial col- 

 umn, and one can easily make out that the vertebral segments are only 

 just distinctly formed. The part of Molacanthus which is comparable 

 with what is the tail of the young and adult of Mola is very rudi- 

 mentary; and since we now know that the volume of this part of Mola 

 actually increases with the advance of age, we have here a remarkable 

 instance of post-larval effort to regenerate a i)art which has degenerated 

 and been absorbed during the larval period of existence, as we have 

 already pointed out in another part of this paper. The figure of the 

 caudal skeleton ot Molaca7ithus in place shows that it is an exceedingly 

 small part of the whole fish, while in the young of Mola it becomes 

 wider, and in the adult still wider anteroposteriorly, while it has in- 

 creased greatly in thickness and changed the form of its posterior con- 

 tour. These are some of the facts upon which I rest my thesis that, 

 there actually occurs in Mola a secondary development of the tail, by icMch 

 that organ is, so to spealc, redeveloped from traces of epaxial and hypaxial 

 tissues ichich had not been absorbed at the time the tip of the larcal axis 

 degenerated. Taken altogether, the tail of Mola thus presents us with one 

 of the most interesting chapters known to me in the whole range of 

 vertebrate morphology, and it has been my real purpose to reconcile it 

 with the general theory of the development of the caudal fin developed 

 in this memoir, constituting as it does an extremely aberrant form, 

 which had to be very thoroughly examined with that object in view, and 



