NO.1:| MORPHOLOGY OF CLADOSELACHE. 109 
be emphasized by very early development. The significance 
of the heterocercal tail of Cladoselache is of especial import- 
ance, since this form occurs earlier than Xenacanthid. It 
suggests that the diphycercy of existing gnathostomes might 
be secondarily acquired. 
Cladoselache, in view of its generalized characters, might 
naturally be looked to for more accurate knowledge as to the 
phylogeny of the ancient Elasmobranchs. Fin and tail structure 
would appear to indicate that the ancestors of Xenacanthids 
may not have been widely separated from Cladoselache. 
Especially interesting is the light that Cladoselache gives as 
to what may have been some of the characters of the Elasmo- 
branch stem, from which the Diplacanths and Acanthodians 
may have had their origin. Comparison with Acanthodians 
brings out many points of agreement.t Shape and outward 
proportions were similar, gills were protected by frills of in- 
tegument, eyes were similar in position and in their protecting 
ring of bony plates ; feebleness in axial parts, and characters 
of dermal denticles were common to both; in lateral line a 
correspondence may, with probability, be traced; tail characters 
were at least similar; similar, too, in these forms appear 
the myocommata, whose clearly-marked character in fossils 
might indicate an unusual thickness or compactness of the 
connective tissue which separated the myomeres. Even the 
most bizarre characters of Acanthodian, lateral fin spines, are 
not altogether incomparable, especially when the evolution of 
the Acanthodian paired fins is taken into account as explained 
by Smith Woodward? Thus in this fin the clustering of 
the unjointed radialia to the anterior fin margin is not unsug- 
gestive of fusion into a single plate, especially as it would 
appear that the lateral fins functioned as balancers, were 
incapable of great upward or downward motion. Fusion of 
the anterior radials might result in a sturdy spine, while 
degeneration and fusion of the posterior radials, which would 
leave a free fin web, might be the origin of the basal 
1 General affinities to Elasmobranchs have been summarized by Reis (Sitz.-Ber. 
d. Gesell. naturforsch. Freunde, Berlin, Nov. 15, 1892). 
2 Ref. 8, vol. II, p..5; 
