214 Notes on Acanthodian Sharks 
plates with the other dermal defenses. in Acanthodians, that there 
exists an evolutional range in the shape, size, and character of these 
structures; from the small denticles of the trunk of the fish we may 
trace transitional forms to the enlarged denticles which margin the 
structures of the lateral line, to those which constitute the dermal head- 
roof, and finally, to those which margin the eye; the last sometimes 
seven or eight in number, instead of the four usually described. That 
the dermal denticles of the trunk may in themselves be subject to an 
evolutional range of forms is also evident; in certain species and genera 
these denticles may acquire the quadrate margins (Figs. 83 and 34), 
which suggest the bony plates of ganoids (fishes with which Agassiz and 
others associated them), others, on the contrary (Fig. 36), may imbri- 
cate their margins to such a degree as to suggest superficially the scales 
of teleosts. Moreover, prominent contour lines may be defended with 
rows of enlarged dermal denticles (already noted by Smith Woodward) 
to such a degree as to suggest the analogous scales of sturgeons or siluroids. 
Thus, in the specimen (Climatius scutiger), pictured in Fig. 14, en- 
larged denticles, shown at 7 s, form a ridge between the head and the 
dorsal fin. In such a ridge as many as ten successive elements may be 
counted.” 
A final word regarding the head-roofing denticles. In some forms 
they may be small and tubercular, in others large and tubercular;” in 
still other forms they may be flattened and closely compacted.” 
Vertebral Column.—tThe vertebral axis of Acanthodians is known to 
retain its notochordal condition and the arrangement of neural and hemal 
spines has already been figured by several authors, Reis notably. Speci- 
mens, however, are rare in which these axial relations may be determined, 
and I might refer in this regard to a specimen of Ischnacanthus gracilis * 
(Fig. 16). Here the neural and hemal arches appear as distinct (meta- 
meral?) elements; as in Cladoselachus there are no interneurals, and 
there is no evidence that the arches approached closely to the surface of 
the body (7. ¢., are not connected with the bases of fins). A remarkable 
condition, on the other hand, is shown in a specimen of Jschnacanthus 
gracilis (Fig. 17), (counterpart of Powrie’s type, No. 251, Edinburgh 
Collection), in which appears a series of superficial elements, 6, in front 
10 Specimens in Brit. Mus., P. 6955-56. 
“Cf. specimens of Parexus recurvus and falcatus, in Edinburgh. 
“2 Cf. Huthacanthus macnicoli, Edinburgh. 
13 No. 334 in the Edinburgh collection. 
