58 Day— On Acrodus. 
The chief grounds upon which this separation was made ap- 
pear to have been, firstly, differences of form of the teeth of 
the two groups; secondly, an assumed diversity, in kind, of 
their structure; and, thirdly, a difference in the degree of 
variation of the teeth upon similar jaws. ‘The most important 
external character, given by Agassiz, of a Cestraciont tooth is 
its depressed and expanded form, fitted for crushing hard sub- 
stances ;* of that of a Hybodont, on the other hand, the cha- 
racteristic is the presence of a greater or less number of cones, 
of which the median is the most elevated, adapting it to retain 
prey when seized.f These differences of form are associated 
with a different arrangement of the enamel which covers the 
teeth ; and upon this, and a co-existent diversity in the in- 
ternal arrangement of the dental elements, Agassiz lays much 
stress.t Such differences, however, as we now have proof, do 
not amount to differences in ind, but are merely of degree; 
and if it can be shown that the external characters of Acrodus 
pass gradually into those of Hybodus, when traced through a 
series of teeth from the same mouth, we may admit, even 
without a special examination, that the internal structure 
would likewise be graduated from the character of one extreme 
form to that of the other. The third argument is based upon 
the fact, that the teeth in the mouth of some Hybodi differ less 
amongst themselves than do those upon the palates of some 
Acrodi;§ but this reasoning is altogether useless, except to 
support the generic distinction, if it be proved that the teeth of 
the two groups are but modifications of the same type. Another 
item of negative evidence that perhaps influenced the judg- 
ment of Agassiz upon this point was that the dorsal spines, or 
‘ Ichthyodorulites, of Acrodus were still unknown;{ the 
possibility of such spines showing a close affinity to Hybodus 
being thus ignored. 
We have now fortunately sufficient evidence to justify us in 
attempting to form a clearer idea of the relationship that existed 
between these two genera; and this evidence lies in a complete 
* Agassiz, ‘Poissons Fossiles,’ vol. iii. p. 159. + Ibid. pp. 178, 179. 
{ Ibid. p. 139, and again at p. 207, where, after speaking of the internal struc- 
ture of Hybodont teeth, he goes on to say: ‘Cette structure des dents s’oppose 
comme on le yoit, au rapprochement que M. Owen’ (in his ‘ Odontography’) ‘a tenté 
entre les dents des Hybodontes et celles des Cestraciontes, en effet les couronnes 
plates qui distinguent les dents des Cestraciontes et qui font des instruments 
propres 4 broyer la nourriture n’ont rien de commun avec Jes couronnes élevées et 
coniques des Hybodontes, qui quoique obtus dans quelques, espéces étaient évidem- 
ment destinées 4 saisir et 4 retenir une proie.’ It must be remembered, however, 
that, when Agassiz made this generalization, he had classed as a Hybodont tooth 
the cephalic spine of Hybodus. Ibid. p. 208. 
§ Ibid. pp. 141 and 182. { Ibid, p. 140. 
