OF THE PAIRED FINS OF ELASMOBRANCHS. 65 



the skeleton of the limb of a nearly ripe embryo). The 

 front end of the anterior basal cartilage becomes segmented 

 oflP as a propterygium {pp), bearing a single fin-ray^ leaving 

 the remainder of the cartilage as a mesopterygium (mes). 

 The remainder of the now considerably segmented fin-rays 

 are borne by the metapterygium. 



General Conclusions. — From the above observations, con- 

 clusions of a positive kind may be drawn as to the primitive 

 structure of the skeleton ; and the observations have also, it 

 appears to me, important bearings on the theories of my 

 predecessors in this line of investigation. 



The most obvious of the positive conclusions is to the 

 effect that the embryonic skeleton of the paired fins consists 

 of a series of parallel rays similar to those of the unpaired 

 fins. These rays support the soft parts of the fins, which 

 have the form of a longitudinal ridge; and they are con- 

 tinuous at their base with a longitudinal bar. This bar, 

 from its position at the base of the fin, can clearly never 

 have been a medium axis with the rays on both sides. It 

 becomes the basipterygium in the pelvic fin, which retains 

 its embryonic structure much more completely than the 

 pectoral fin; and the metapterygium in the pectoral fin. 

 The metapterygium of the pectoral fin is thus clearly homo- 

 logous with the basipterygium of the pelvic fin, as originally 

 supposed by Gegenbaur, and has since been maintained by 

 Mivart. The propterygium and mesopterygium are obviously 

 relatively unimportant parts of the skeleton as compared 

 with the metapterygium. 



My observations on the development of the skeleton of 

 fins certainly do not of themselves demonstrate that the 

 paired fins are remnants of a once continuous lateral fin; 

 but they support this view in that they show the primitive 

 skeleton of the fins to have exactly the character which 

 might have been anticipated if the paired fins had originated 

 from a continuous lateral fin. The longitudinal bar of the 

 paired fins is believed by both Thacher and Mivart to be 

 due to the coalescence of the bases of the primitively 

 independent rays of which they believe the fin to have been 

 originally composed. This view is probable enough in 

 itself, and is rendered more so by the fact, pointed out by 

 Mivart, that a longitudinal bar supporting the cartilaginous 

 rays or unpaired tins is occasionally formed ; but there is no 

 trace in the embryos of the Scyllium of the bar in question 

 being formed by the coalescence of rays, though the fact of 

 its being perfectly continuous with the basis of the fin rays 

 is somewhat in favour of such coalescence. 



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