424 OSBURN 



caudal fins there is not the least indication of such a close 

 relation of the rays to the vertebral processes as we should 

 expect from the above view of their origin. In the lowest 

 sharks, especially in such forms as do not possess fin spines, 

 the rays of the dorsal and anal fins are usually widely separated 

 from the axial skeleton, according to the present studies on 

 many species. In the ancient fossil shark Cladoselache, also, 

 the rays of the dorsal fin are widely separated from the vertebral 

 column. 



(2) In ontogeny the skeleton of the unpaired fin is plainly 

 developed from a plate of thickened mesenchyme which first 

 appears next to the ectoderm. 



(3) During early development the unpaired fin skeleton is 

 never in contact with the axial skeleton, as shown by my studies 

 of Cestracion, Chlamydoselachus, Spinax, Mustelus, and others. 



(4) The only exception to the last statement, and the only 

 case where corresponding rays are known to come into contact 

 with vertebral processes, are found in the inferior caudal fin, 

 and even here there are frequent discrepancies. This fin is 

 then an exception to the rule, and if all the unpaired fins have 

 had a similar origin, as seems probable, we must explain the con- 

 dition in this fin as due to secondary fusion of the rays with 

 hasmal spines to secure better support. On account of their 

 mechanical relation to the ventral lobe of the caudal fin, which 

 is the chief organ of propulsion, these rays stand in need of 

 just such support. (This is the part of the fin which becomes 

 the functional caudal in Teleosts.) Examples of parallel cases 

 are the dorsal spines of sharks (undoubted secondary struc- 

 tures), which, in order to secure firmer support, have become 

 secondarily attached to the axial skeleton, and the superior 

 rays of the secondarily diphycercal tail of Dipnoi. 



II. With regard to the comparison of the girdles of the 

 paired fins with the basalia of the unpaired fins a number of 

 facts present themselves. 



(1) The girdles exhibit so much variation in form that they 

 show themselves to be adaptive structures such as the basalia of 

 unpaired fins are admitted to be. 



