No. 430.] PAIRED LIMBS OF VERTEBRATES. 773 
are still doubtful, the present knowledge of them having been 
based upon single, imperfectly preserved, and fragmentary 
specimens. The conditions in Cladoselache, on the other hand, 
are known from upward of thirty specimens, some of which are 
admirably preserved.! In conclusion, if the Gegenbaurian 
adherents are to contest the sum and substance of the paleon- 
tological evidence, they have, it seems to me, the difficult task 
of demonstrating (1) that acanthodians, the most extensive 
group of Paleozoic sharks, do not possess pleuropterygia ; (2) that 
Cladoselache, in spite of its earlier occurrence and peculiar 
structures, is not as archaic a type of shark as Xenacanthus ; 
and (3) that the *archipterygium " formed in the unpaired fins 
as the terminal member of a series whose original structures 
were of fin-fold character, is formed in exactly the opposite way 
from the archipterygium in the paired fin. Until these contra- 
dictions have been disposed of, no evidence based upon recent 
forms can, I believe, attack the main trenches óf the lateral- 
fold theory. 
Postscriptum. — Objections which have been urged to the 
foregoing conclusions. 
As far as I am aware, no objections have been raised to the 
conclusion that the paired fins of the acanthodians represent 
definite lateral dermal folds, whatever may be regarded as the 
homologues of the supporting elements of these fins. (Thus 
Fritsch endeavors to find in the pulp canals of the anterior fin 
spines rudiments of an entire biserial archipterygium (Fauna d. 
Gaskohle Formations Bóhmens, Bd. III, p. 71). (As a corollary 
to this suggestion, I take it, the fin spines of 2// acanthodian 
fins are equivalent to archipterygia, since the spines are sim- 
ilar in structure in all fins!) Cladoselache, however, has been 
subjected to adverse criticism by several advocates of Gegen- 
baur’s hypothesis. Professor Jaekel, the first of these critics, is 
the only one who appears to have had the opportunity of exam- 
ining actual specimens of this form, for during his visit to 
1 Sections of the muscle plates have been found to show transverse striz in 
the fossilized muscle cells. Their description will shortly be published. 
