Day—On Acrodus. 63 
the grounds of the relationship have not, that I am aware of, 
been definitely stated. 
From an examination of various remains of Hybodus now 
accessible, it is evident that, although the teeth in this genus do 
not differ so much, upon the posterior and anterior portions of 
the jaw, as they do in Acrodus or Cestracion, yet there is always 
some amount of variation ;* and it appears to me that the more 
elongated the cones are, the less the variation in the entire 
series of teeth in any species; therefore this difference again 
is only one of degree. From heads of Hybodus basanus, 
Egerton, we learn that Hybodus possessed an expanded jaw, 
similar to that indicated by our figure of Acrodus; nor must 
much stress be laid upon the presence of cephalic spines with 
the former genus and their never having been found with the 
latter, since we have them positively associated only with a few 
species of Hybodus, of which the remains are far more frequently 
met with than those of any Acrodus. In fact, regarding these 
two genera as one group, such a group would be for convenience 
divisible by dental characters into three sub-divisions; the 
first, with very elongated cones, represented by Hybodus 
basanus; the second, with the cones more obtuse, by H. De- 
labechet; and the third, almost or altogether wanting conical 
elevations, by Acrodus nobilis; A. Anningie would then be in- 
termediate in characters between the second and third groups, 
and, by thus intervening, would tend to show the artificiality 
of the whole arrangement. 
Understanding now the close affinity between the two 
genera, I will add a few words upon their affinities to existing 
forms. Agassiz indicated a close affinity between Acrodus and 
Cestracion, from a consideration of the structure of their teeth ; 
but as his own classification, viewed by the light now obtained, 
shows, the resemblance, somewhat close in typical Acrodi, 
becomes altogether lost as we pass to the true Hybodonts, 
which, on the same consideration, show a relationship to the 
ordinary Sharks. Again, the mouths of Hybodonts (that is, 
including Acrodus) were not only very different in form from that 
of the Port Jackson Shark, but they differed from the mouth 
1863, p. 243. On the other hand, Pictet, in his ‘Paléontologie,’ 1853-7, vol. ii. 
pp. 254 and 260, retains the error of classifying the two in different families ; as is 
likewise done in Morris’s ‘Catalogue of British Fossils,’ on the authority of Pro- 
fessor Owen’s ‘ Lectures on Comparative Anatomy,’ vol. ii. p.47. A less compre- 
hensible oversight occurs in Owen’s ‘ Paleontology,’ where a side view of the head of 
Myliobates is apparently copied from Agassiz and referred to in the text as that of 
Cestracion Philippi, p. 106. 
* The most marked exception to this generalization is Hybodus basanus, de- 
scribed by Sir P. Egerton, Quart. Jour. Geol. Soe., vol. i. p. 197. 
