FINS OF ELASMOBRANCHS. 



477 



Fig. 6. 



Altgether, valuable as are such suggestions as hints for the direction of inquiry, 

 I do not think we are yet in a position to do more than guess at the exact form and 

 precise mode of origin of the archipterygium. 



There is also the objection that something may be said in favour of the line of the 

 propterygium having furnished the air-breathing limb-axis. 



Before entering upon this point, however, it may be well to review Professor 

 Huxley's i special interpretations of fin-homologies. 



In the first place, he regards the Notidanus-Tpectoral as the least modified form of 

 Elasmobranch pectoral, that in which the Ceratodios type is least departed from. The 

 mesopterygium he regards as the remains of the shrunken axis ; the propterygium he 

 appears to deem a modified preaxial ray ; and the metapterygium he considers ^ as 

 " formed by the coalescence of the axial ends of the pectoral rays." 



He does not, however, explicitly refer to the small cartilage called by Gegenbaur 

 propterygium, and figured by him ^ and found by me (Plate LXXV. fig. 3, a) to be 

 divided ofl" from the bases of the preaxial rays by a preaxiad extension of the meso- 

 pterygium. Moreover I did not find the most preaxial ray much " broader than the 

 others," or " two-jointed," but distinctly divided into four segments, while Gegenbaur 

 represents it in K griseus'^ as much narrower than the 

 others, and with only one joint. In N. cinereus^ he repre- 

 sents it with five segments and a very small, still more pre- 

 axiad cartilage annexed to it very near its base. Evidently, 

 then, there is much individual variation in these parts, as it 

 might well be expected a priori that there would be. I 

 cannot, therefore, regard the above explanation of Notidanus 

 by Ceratodus as more than an ingenious speculation. 



In Cestracion " we have, as we have seen, a great cartilage 

 which evidently represents both the pro- and mesopterygium 

 undifferentiated, or coalesced; and we have evidence of the 

 great plasticity of those structures in the appearance of three 

 large cartilages connected (two directly and the third by these 

 two) with the large compound cartilage. 



That this great cartilage is so complex in nature is made 

 evident, I think, by the pectoral of Scymnus lichia ', in which 

 one great cartilaginous mass must answer to the pro-, meso-, 

 and metapterygium, all three. 



Profesor Huxley next speaks of Scyllium ^ which has a greatly reduced mesoptery- 

 gium (still considered, of course, by Professor Huxley as the reduced limb-axis) and a 



1 p 2. S. 186. ' L. c. p. 50. ' Untersuch. ii. plate is. figs. 1 & 2. 



* L.c. fig. 1. ' L. c. fig. 2. " Huxley, I. c. fig. 11, page 51, and Gegenbaur, pi. is. fig. 3. 



' Gegenbaur, plate ix. fig. 9. ° L. c.fig. 10, p. 48. 



Pectoral fin of Scymnus 



lichia. 



