188 Origin of the Paired Limbs of Vertebrates 
THE ORIGIN OF THE GIRDLES. 
While upon this point it must be admitted that the evidence is not as 
complete as we would desire it to be, and while in a few respects it is 
even conflicting, yet, reviewing the whole matter carefully, the origin of 
the girdle may, I believe, be traced to the supporting elements of the fin. 
The raison d’étre of the girdle is, naturally, the necessity for a solid 
base in the soft body-wall, against which the remainder of the fin skele- 
ton may brace itself. The rays of the unpaired fins may, when necessary, 
find support against the axial skeleton, but as the paired fins are situated 
support is impossible unless developed for that special purpose. Is such 
a development impossible of conception or unparalleled in morphology ? 
For answer we will cite the extradigital cartilages which support the 
swimming membranes of the seal, the extension of the cartilage area, 
and the development of accessory digits in Cetacea and Ichthyosauria 
for the same purpose (Kiikenthal, 90; Osburn, o6a) the calcar of the 
bat’s wing, auricular and nasal cartilages, etc., etc., not to mention the 
visceral skeleton which must at some time have arisen in situ for the 
support of the gills. 
Is there anything similar to this process in the unpaired fins? Un- 
doubtedly, for in those forms in which, in the adult condition, the fin 
skeleton rests against the axial skeleton for support (Spinax, Cestracion, 
Acanthias, etc.), the embryonic fin skeleton is at first developed sepa- 
rately from the spinal column and by later growth comes into contact 
with it. In fact, in the unpaired fins of nearly all sharks there is 
more or less development of the proximal portion of the radials into 
basalia. The pelvic plate or girdle of Chlamydoselachus is certainly 
most similar to such basaha of the median fins. It serves as a direct 
support for half of the rays of the fin (without the intervention of any 
other cartilages), and the two girdles meet at the mid-ventral line for 
mutual support (Fig. 20), just as the basalia of certain median fins grow 
into contact with the axial skeleton for the same reason. The pelvic 
basalia or girdles are not in contact when they first appear but meet as 
development proceeds. The pelvic skeleton is not very unlike that of 
the anal if we suppose a similar amount of “fusion” to take place in 
the latter. 
Is it a mere matter of coincidence that this simple condition of the 
pelvic fin should appear in a species which gives so many other evi- 
dences of great antiquity? We think not, especially when we take into 
consideration the evidence from Cladoselache (the oldest fossil shark in 
which the pelvic structures are known (see Dean, 94, Pl. VII, Fig. 2, 
