Day— On Acrodus. 57 
structure of any one scaly type by these feathered vertebrates ; 
those reptilian qualities and excellencies which are best and 
highest have become theirs; but how much more! ‘This exal- 
tation of the ‘ Sauropsidan’ or oviparous type by the substitu- 
tion of feathers for scales, wings for paws, warm blood for cold, 
intelligence for stupidity, and what is lovely instead of loath- 
someness,—this sudden glorification of the vertebrate form is 
one of the great wonders of Nature. 
Ill. On Acropus Anninci#, AGass.; WITH REMARKS UPON THE 
AFFINITIES OF THE GENERA Acropus AND Hysopus. 
By E. C. H. Day, F.G:S. 
[Plates HI. and IV.] 
EW amongst the Fish-remains preserved to us in the 
Secondary rocks are more commonly met with than those 
of Sharks of the genera Acrodus* and Hybodus;{ yet, not- 
withstanding the frequency of their occurrence, we have but 
little exact knowledge of the form and affinities of the fish to 
which these remains belonged. ‘Their cartilaginous skeletons 
have, excepting a few fragments, altogether perished; and 
it is quite impossible to guess at their outlines, undefined 
as these were either by scales or hard plates. Nay, more, 
the remains that are known of these extinct forms present 
such great differences from the corresponding structures of 
living “fish, that, although a relationship to a single existing 
genus has long been indicated, the degree of that affinity is 
still very uncer rtain. 
Of the two genera, the remains of Acrodus are the less fre- 
quently met with ; and its structure is, in consequence, the less 
known. At the time that Agassiz wrote his celebrated work 
upon fossil fishes,{ detached teeth and one or two incomplete 
palates, or groups of teeth associated in their normal order, and 
some traces of the shagreen, or skin of the shark, were all the 
materials at his disposal for determining the characters of the 
genus. Relying upon these, he referred Acrodus and Hybodus 
to different families, assigning the former to the Cestracionts,§ 
of which the Port Jackson Shark, Cestracion Philippi, is one 
of two existing examples, and making the latter the type of 
a new family, the Hybodonts.| 
* Agassiz, 1838. t Ibid. 1837. 
t The volume containing those that form the subject of the present paper was 
published 1833-43. 
§ Agassiz, ‘Poissons Fossiles,’ vol. iii. p. 139. || Ibid. p. 206, 
