CTENACANTHUS, A SPINE OF HYBODUS. 149 
rare to obtain t 
entire; it is finely and. lo — y_ striated; .the division 
is marked by a very distinct i of demarcation, which is some- 
the base of the spine is open, a very deep furrow extend- 
ing deeply into the body; as this furrow proceeds upwards it 
becomes closed in posteriorly and forms an oval cavity, this oval 
e ter is in many cases lost, for numbers of the larger spines, 
having necessarily large cavities, have been crushed in by the 
fo cea pressure ; the cavity extends nearly to the apical 
extremity. The exposed portion of the spine is rounded a 
rior 
each other by grooves of nearly the same dimensions as the 
ridges ; the grooves and ridges run parallel with the anterior 
border, but as they approach the apex they become fewer in 
some case: to. 
CE they are mae ae or even longitudinally 
striated. The ridges, according — are 
generally supposed to be tuberculated or erenelated, but 
1s evidently a mistake, for among the specimens I have observed, 
I on sa a gam six in — = — were perfectly smooth 
rounded a cases they varied from 
of the spine to being aaa denticulated on the posterior ridges 
of the lateral surfaces only, the non-tubereuated ridges being 
20 in Agassiz’ 3rd volume, at resemble them. 
A careful comparison of the a above descriptions of Ctenacan- 
thus and H Hybodus will show that the so-called spines of Ctena- 
canthus from the spines of Hybodus in very few particulars, 
Si tect na secrecy, first, that the posterior surface of the former 
of that » ear is is general y observed 
in Hybodus, but Agassiz pvens taaeg s Eis: third 
