344 I'^DAVIN S. GOODRICH. 



figs. 1, 2.5). It is also borne out by a comparison of tlie range 

 of extension of the fins in various genera; for instance, Avhilst 

 the paired fins occupy segments 5-23 and 47-65 in Zj^goena, 

 they occupy segments 2-19 and 29-50 in Heptanchus and 

 segments 2-15 and 19-37 in Chimaera (Braus). 



The conclusion that every trunk segment is capable 

 of producing muscular, nervous, and skeletal ele- 

 ments of the median dorsal fin is likewise reached on 

 examining the structure and development of that organ. It 

 is well known that a more or less perfectly continuous dorsal 

 fin still exists in many modern Teleostei, and was present 

 in many extinct forms (Dipnoi, Pleuracanthus). I shall be 

 able to show below (p. 353) that the muscle-buds giving rise 

 to the widely separated adult dorsal fins of Scyllium form a 

 continuous series in the embryo. 



Every trunk-segment, then, is potentially able to produce 

 paired and unpaired " fin-elements.'^ But, even if the ances- 

 tral Gnathostome was provided with continuous-paired fin- 

 folds, the position of the paired limbs of vertebrates can not 

 be accounted for merely on the supposition that these folds 

 have survived in this or that region. The paired limbs have 

 certainly altered in position since they were first established 

 with regard to the numerical order of the segments they 

 occupy. In fact, it is clear that a perpetual shifting of 

 the position of the limbs has taken place in all classes of 

 Gnathostome vertebrates. 



It seems to be often held that these changes of position are 

 brought about either by the actual shifting or migration of 

 the limb from one place to another, or by the excalation and 

 intercalation of segments. We cannot, in this paper, enter 

 into a discussion as to the origin and significance of meta- 

 meric segmentation in vertebrates; but something must be 

 said about the theory of excalation and intercalation, strongly 

 supported many years ago by v. Jhering (18). Already it 

 has been so severely and successfully attacked by Furbringer 

 (11) that it can be very shortly dismissed. 



In the case of the })elvic fins of Teleosts, for iustaiRH-, tlici-e 



