220 Notes on Acanthodian Sharks 
distinctly cladoselachian in type. Taxonomically, therefore, Cladose- 
lachians and Acanthodians should be more closely associated than should, 
for example, Cladoselachians and Xenacanths, or Acanthodians and Xena- 
canths. We are thus led to suggest again” that a group such as defined 
by Pleuropterygii (sharks with fins of fin-fold type), would justly in- 
clude both Cladoselachians and Acanthodians. ‘This group, however, 
might more accurately be regarded as of superordinal rather than of 
ordinal rank, and ordinal rank would thus remain in the groups 
Cladoselachia and Acanthodia. 
The question as to which groups, Acanthodian or Cladoselachid has 
retained the more primitive ‘character is indicated in the accompanying 
table (v,p.221). In the great majority of structures one can only con- 
clude that the Acanthodians have passed through a stage in evolution 
which is best represented by the Cladoselachian. In one regard only 
(apart from the matter of size) does the latter appear the more special- 
ized, 7. e., in the unsegmented character of the mandibular arch (v, how- 
ever, supra). 
If these things be true, it may next be queried why is it that the more 
specialized group is known from an earlier horizon? For certainly the 
appearance of the Acanthodians in the upper Silurian (as against the 
Cladoselachian in the upper Devonian) is in general evidence of the 
greater primitiveness of their group. And this is in truth a question 
which can be answered only by the time-worn appeal to the defectiveness 
of the paleontological record, noting especially in this regard that the 
soft structures of the Cladoselachians would be less apt to be preserved 
than the hard structures of the Acanthodians. We may, however, safely 
predict that from the earliest Acanthodian horizon there will be dis- 
covered forms which will represent the ancestors of all of the early groups 
of sharks. And we may predict with almost the same degree of security 
that these forms will be found to picture the Cladoselachian in essential 
characters. For the Acanthodians, as we at present know them,” are 
obviously too specialized to have represented the ancestors of the line of 
Cladoselachians. 
The causes of the extinction of the Acanthodians can, I believe, be 
suggested with a fair degree of probability. It is evident, of course, 
that the fin characters of this group premise great capability on the part 
of these fishes to dart forward, 7. e., in direct lines, a type of movement 
especially valuable when a definite kind of food is to be secured. On the 
> Cf. J. Morph., Vol. IX, pp. 110-111. 
*° Cf. the accompanying table. 
