150 CTENACANTHUS, A SPINE OF HYBODUS. 
but I cannot accept this opinion, for the spines present every 
variation between the two extremes, and both varieties are found 
the same microscopical structure. For the purposes of compari- 
son with the spines of Hybodus, I have of course chosen the 
most typical specimens of Ctenacanthus. No. I e upper 
of a spine of Ctenacanthus; on its lower part it 1 
exposed portion. A transverse section through No. I shows the 
shape of the internal cavity and the slight concavity of the 
with the exception of being larger, there 1s 
thi i nd t ion 0 
