414 The American Naturalist. [May, 
column through the intermediation of the supporting elements in 
certain cases. In a review of Zittel’s Paleontology of Fishes, 
published in 1887,” I applied the facts of this part of the struc- 
D 
Fr 


A. 

Fic. 6—Diagram the elementary constitution of the fins; D, dorsal fin; 
fin ; Pv, pectoral and ventral fins; Sf, neural spine ; Ax, axonost; Ba, baseost ; 
Fr, PaE Cv, chevron bone; Sg, shoulder girdle, 
ture to the question of taxonomy and phylogeny more fully, es- 
tablishing the subclass Rhipidopterygia, and several orders, on the 
varying characters of the median fin supports. Some additions 
were made to this system in a “Synopsis of the Families of the 
Vertebrata,” published in 1890 The basis of this analysis is 
the hypothesis above cited, that the fins represent longitudinal 
folds of the integument, within which have been developed rods 
which form a framework connecting the free edges of the folds 
with the vertebral column. These rods may have been primi- 
tively undivided neural spines, but in the oldest forms known to 
us two segments exist distad to the neural spine. These I have 
termed the axial and basal elements, or the axonost and the bas- 
eost; the latter supporting the fin-rays when they exist. Fin- 
78 AMERICAN NATURALIST, 1887, p. 
29 AMERICAN NATURALIST, 1889, Soe published March, 1899. 


