174 Origin of the Paired Limbs of Vertebrates 
writer in taking up the present studies, to discuss thoroughly the em- 
bryological work on the origin of limbs, but the recent able papers of 
Professor Braus, though taking an opposite view of the question, have 
dealt so thoroughly with the literature as to make such a discussion no 
longer necessary. In May, 1906, the writer paralleled the arguments 
of the fin-fold and gill-arch theorists as a preliminary to the present 
paper. In the month following there appeared the admirable paper of 
Professor Goodrich, 06, in which he arrives at conclusions very similar 
to my own, based chiefly upon a study of Scylhum. The present writer 
has examined especially Cestracion and Chlamydoselachus. However, 
under the behef that embryological evidence from the last two forms 
mentioned should have special consideration on account of their primitive 
position among the genera of Selachii, and for the reason that certain 
heretofore unpublished facts have been brought to light during the 
pursuit of my studies, the publication of my results seems desirable. 
I may state at once that as far as my own studies overlap those of 
Goodrich they are entirely corroborative of his work, and where they do 
not cover the same ground they are quite confirmatory of the fin-fold 
point of view. It should be said that our work was carried on in parallel 
lines and without any knowledge of the other’s research. This, in itself, 
must add weight to our results. As will be seen, the evidence indicates 
strongly that the unpaired fins have originated as external structures 
entirely independent of the axial skeleton, and the remarkable similarity 
of the paired to the unpaired fins in mode of development can only be 
explained on the ground of similarity of origin. The failure of Braus, 
oga, to find in Spinax any marked similarity between these fins is ex- 
plicable only under the supposition that he did not make a careful com- 
parison of the earher stages in the development of the various component 
structures of these organs, or that he did not possess the proper material 
for such comparison. As a matter of fact, the unpaired fins of Spinax 
are very much concentrated (the anal is entirely wanting) and the early 
stages of development seem to be passed through much more rapidly 
than in Cestracion, though the dorsal fins of the latter species seem, in 
the adult, to show nearly as much concentration as those of Spinax. 
Zodlogists generally agree in holding that the sharks approach more 
nearly than any other group the original condition of the gnathostome 
vertebrates, and it is to this group that we naturally look for embryo- 
logical evidence bearing on the origin of the paired limbs. Naturally, 
also, we look for the best embryological evidence as to the origin of fins 
among those sharks which on anatomical and paleontological grounds 
