CTENACANTHUS, A SPINE OF HYBODUS. 151 
at its ¢ Then again, in C. major - 4, fig. 3) the 
Scabeick paki “est ovale et arrondie.” These descriptions 
and figures are manifestly in scammits ; it may be that the speci- 
mens he examined -were either crushed —— or else much 
The Seep 9 sells of these — is very little known, 
the only author that I am acquainted with who attempts to 
describe it is Ageshis ; “i ne “ Poissons Fossiles,” tome 3, he 
gives two engravings (tab. A., figs. 8 and 9) of the structure of 
Hybodus reticulatus, they are, however, poor representations ; 
h 
précédent (Datonacoestivan) que par le aid grand nombre de 
couches concentriques qui entourent la cavi ite médian » et t dans 
mais en pétit n mbre. Les lisiéres saben qui se trouvent 
entre la dentine cide dont les canaux secondaires sont entourés 
I know many points of resemblance to Ctenacanthus an - 
fore to Hybodus. Agassiz does not refer to he structure of 
Ctenacanthus. Having therefore no authority upon whom I can 
depend for a correct account of the minute structure of Hybo- 
dus and oe shall fhe § bce Boome sections that I 
ve had made in my presen a revent a great 
amount ~ so. I will state ie cite tke the structures of 
these two spines are very similar, for under the mncrOnCD pe it is 
well nigh — to say which is which. In the 
sections uired, for if too thick nothing can be 
seen but the declines of the larger canals and dark tissue be- 
( rawings gassiz ; if too 
thin, then the calcigerous — more © especialy the finer — 
are broken or ground aw ay. The x eae 
