104 DEAN. [Von. IX. 
like bar which passes over and appears to be attached to its 
next neighbor, thus forming a cartilaginous margin to the fin 
web. The rays show a trace (?) of but a single basal support. 
It is evident that this material is in no way sufficient for 
generalizations, — e.g., as to the beginnings of unpaired fins, 
or as to the troubled question of dorsal spines in Cladoselache. 
One is, however, tempted to comment on the mode of 
strengthening the distal web margin, which Polypterus has 
so aptly specialized, in all probability as a neomorph. So, 
too, the foremost rays in their process of clustering might 
readily be adduced to prove the mode of origin of a dorsal 
spine. Or the hollow nature of the hindermost (and there- 
fore the least modified) rays might be emphasized as typifying 
the most ancient form of fin support. 
This specimen will become of value when others are found ; 
—jin the present paper it is described as the only known 
remains of an unpaired fin. 
THE SHAGREEN AND DERMAL DEFENCES. 
Palaeozoic sharks seem as a rule to have been richly pro- 
vided with dermal defences; many and characteristic spines 
were evolved, specialized in their characters to a remarkable 
degree, as in the hook-like head spines of Hybodus and 
Acrodus, or in the fin spines of Acanthodes or Diplacanthus ; 
shagreen was often stout, tuberculate and sculptured, at 
times ‘ganoid’ in its outward characters, varying often in 
its coarseness of texture in different regions. Xenacanthids, 
as well as Chondrenchelys, however, appear to have been 
shagreened with the finest of denticles, a deficiency in dermal 
defences perhaps of a secondary nature, since in the former 
instance at least a median spine was present. In Cladoselache, 
it is to be noted that fineness of shagreen texture is unaccom- 
panied by spines, —as far at least as can be positively judged 
from the present material. In this form the denticles become 
larger in size at the sides of the head, in the region of the 
jaw angle (suggestive, perhaps, of Ch/amydoselache) and in a 
marked way on the anterior margin of fins and tail. The 
denticles are usually lozenge-shaped, varying in shape and 
