THE ORIGIN OF VERTEBRATE LIMBS 425 



(2) In the fossil Cladoselache we have the evidence that 

 the pelvic girdle was formed in the same way as the basalia of 

 unpaired fins, — indeed it is in the same condition as many of 

 the unpaired fins of modern sharks. 



(3) In the Notidanidas, which are without doubt the lowest 

 and most primitive of recent sharks, the pelvic girdle is merely 

 a flat plate, not more complicated in form than the basalia of 

 many unpaired fins, and in Chlamydoselachus twelve of the 

 twenty-five rays of the pelvic fin attach directly to the girdle, 

 thus indicating its primitive position in the category of basalia. 



(4) In the more primitive Ganoids the skeleton of the paired 

 fins has a close resemblance to that of the unpaired, as Thacher 

 and Mivart demonstrated long ago. Regan ('04) has recently 

 brought forward a remarkably clear case in Psephurus gladius 

 in which the series formed by the anal, pelvic, and pectoral is 

 most convincing. The pelvic resembles the anal even more 

 than it does the pectoral. 



III. The presence of post-axial rays in the pectoral fins of 

 the fossil Pleuracanthus, and to a limited extent in modern 

 selachians, is held by the gill-arch theorists to prove the "primi- 

 tive biseriality" of the paired fin skeleton. Opposed to this 

 conclusion we have the facts : 



(1) The fossil Pleuropterygidae, Acanthodidae, and Diplacan- 

 thidas, all of which occur in older strata than does Pleuracanthus r 

 have the fins all decidedly of the fin-fold type, with not even 

 any opportunity for the presence of post-axial rays. 



(2) Post-axial rays are entirely absent from the pelvic fins 

 of all sharks, modern and ancient (unless we are to accept 

 the questionable cartilages in the mixipterygium of Pleuracan- 

 thus, the homology of which with post-axial fin-rays is at least 

 doubtful.) 



(3) The occurrence of post-axial rays in the pectoral fins of 

 recent sharks is so sporadic and variable, and they are wanting 

 entirely in so many species, that they are better considered as 

 adaptive structures without peculiar phylogenetic significance. 



(4) There are known cases of post-axial rays in the unpaired 

 fins (dorsal of Raja, anal of Heptanchus; also the dorsal of the 

 Devonian Ganoid, Ccelacanthus) , where they have all the 



