BY WILLI.VM A. HASWELL, M.A., BSc. 9 



If these cases of plurality of axes in the limb are cases of 

 atavism, then they may perhaps point back to a common form of 

 fin-skeleton whence the normal specialised fin of Ceratodios and 

 the clieiropterygium of the higher vertebrates may have been 

 derived. The speculations of Prof. Huxley (J. c, p. 56) would 

 however, require little modification to adapt them to such a 

 biaxial or multi- axial type of limb, and it would be idle, in view 

 of the variations which I have shewn the fin-skeleton to present, 

 to offer any further more detailed suggestions in this direction. 



If we were to speculate as to the nature of the earliest fin- 

 skeleton, we should most probably, in view of the structure of 

 the embryonic fin in the Elasmobranchii, come to the conclusion 

 that at first it consisted of a series of detached nodules of 

 cartilage ; that, as muscular action became more definite in 

 direction, these nodules came to be arranged in rows so as to 

 assume the form of numerous parallel, jointed rods of cartilage, 

 which might coalesce in some cases so as to form a continuous 

 plate ; the coalescence of the bases of these jointed rays as the 

 two pairs of fins became differentiated from one another and the 

 proximal part of each fin became narrowed, and the inward 

 growth of this, probably, as Thacker has suggested, formed the 

 hasipterygium. As the breadth of the whole fin became decreased 

 to form the narrow, pointed paddle of Ceratodus^ the jointed rods 

 became approximated not only at their bases but throughout a 

 considerable portion of their length, and their partial coalescence 

 resulted in the formation of a branching structure, the branches 

 of which, as centralisation went on further, came to be arranged 

 on each side of a single stem — the axis of the so-called arehi- 

 pterijgium. 



A study of the soft parts of the fin in Cemtodits points to the 

 same conclusion as the consideration of the skeleton. The 

 pectoral fin of Ceratodus as observed by Mr. E. P. Eamsay, is 

 capable of somewhat complex movements, and to accomplish 

 those the muscles, though simply arranged, are somewhat more 



