T70 THE AMERICAN NATURALIST. [Vor. XXXVI. 
by Cope, is similar to cladoselachids, but remarkable in that its 
hindmost rays apparently coalesce at their bases so as to form 
a stout supporting plate. There is no evidence, however, that 
this projected from the body wall, nor have we any knowledge 
of pelvic or unpaired fins. In xenacanthids, on the other hand, 
many and well-preserved specimens (of two or three species) 
show a biserial archipterygium in the pectoral fin, and this 
condition has been the main paleontological stay of those who 
oppose the fin-fold hypothesis. They lay no emphasis, how- 
ever, on the late appearance of this form (for there is no 
evidence forthcoming that the Coal Measure “ Xenacanthus ” 
had fins like the Permian form), or on the facts that its ven- 
tral fin is zo£ a biserial archipterygium but resembles closely 
the unpaired fins immediately behind it, and that this form has 
already developed a mixipterygium, and has specialized an 
amphistylic attachment of jaw to skull, and that it has largely 
lost its integumental defenses. 
Balancing the matter fairly, I think that we must conclude 
that the weight of the evidence of Paleozoic sharks falls 
emphatically on the side of the longitudinal fin-fold hypoth- 
esis. Indeed the only way, as far as I can see, in which this 
evidence can be denied is by a frank rejection of the well- 
accepted view as to the primitive nature of the earliest elasmo- 
branchs (Proselachii). 
Il. Tue Fin-Fotp CHARACTER OF THE EARLIEST FORMS, 
ACANTHODIANS AND CLADOSELACHIDS. 
The dermal-fold nature of the paired fins of the acantho- 
dians requires little comment. Excepting in a brief note of 
Dr. Fritsch,! no attempt has ever been made to explain them 
in accordance with the theory of Gegenbaur ; that is, that their 
origin is due to outgrowth of elements belonging to serially 
homologous gill bars. Continuous fin folds they certainly are; 
1 Fauna d. Gaskohle Formations Bóhmens (1893, p. 71), in which the spine le 
regarded as the archipterygium. Dr. Jaekel homologizes shoulder girdle to gill 
arch, but does not consider the relations of the fins themselves. Verhandl. 
Deutschen Zool. Gesell. (1899), pp. 256-258. 
