n[ 



432 OSBURN 



girdles, in the case of the paired fins, appear very rapidly. In 

 the unpaired fins the process is identical, except for the girdles. 

 We must insist upon the fact that the rays do not "grow out" 

 of the basalia but that both structures are differentiated in the 

 same way out of the same mesenchyme plate. Similarly the 

 girdles do not grow out of the basalia, nor vice versa. 



VII. As we have seen, the fin fold originates in precisely 

 the same way in paired and unpaired fins. The process of 

 further development is also similar in both. The base of the fin 

 grows relatively slowly, while the body is elongating and the 

 external part of the fin fold is pushing out very rapidly. This 

 results in a concentration of the fin base, in the manner described 

 by Mollier .('93) for the paired fins. The median fins pursue a 

 similar course, though usually the process is not carried to such 

 a degree. The shape taken by the fins during development 

 depends upon what Mollier has termed their "direction of 

 growth." The amount of constriction at the base of the fin 

 is presumably measured by the amount of mobility required 

 of the fin. 



VIII. The so-called "fusions" of rays and basalia, which, 

 in reality, are merely failures to differentiate separately out of 

 the mesenchyme and are not due to the growing together of 

 parts, may occur at any part of the fin skeleton, but according 

 to my observations made on many preparations, as well as on 

 data provided by the plates of various authors, they occur 

 much more commonly at or near the ends of the series of rays. 

 It would seem that mechanical conditions would naturally be 

 most effective in producing them here. The mode of formation 

 of these "fusions" and the manner of their occurrence, both of 

 which are similar in all fins, lead us to conclude that they are 

 of one kind with the larger basalia and that all such sporadic as 

 well as regular cases are produced in adaptation, i. e., to meet 

 the mechanical needs of the particular fin in which-such struct- 

 ures occur. 



IX. While spines are not found in the paired fins of modern 

 sharks, they are to be found in those of some of the oldest 

 fossil forms (Diplacanthidse and Acanthodidse, also Gyracan- 

 thus, Haplacanthus, and Heteracanthus) . This shows the very 



