64 Day— On Acrodus. 
of the latter, which is situated at the extremity of the head, in 
position, being placed under a projecting muzzle, as is the case 
in the majority of Placoids. From various mistaken statements 
on the subject, there exists a very general impression that the 
nearest existing approach to the Ichthyodorulites of Hybodus 
is to be found in the spines of Cestracion. ‘The former have, 
however, a far greater resemblance to the single spine of a 
Chimerotd fish; we see this, not only in the elongated form, 
but in the distinct prolonged tapering root and the two rows of 
curved denticles upon the posterior edges; and this spine, 
marked with fine longitudinal striz, is so hollow that even in 
the recent specimen its slight walls are crushed in, just as we 
find the Ichthyodorulites of the Lias.* In all these points, as 
our figures (Pl. IV., figs. 8 and 9) show, the spines of both 
Hybodus and Chimera differ from those of Cestracion. This 
similarity between the single spine of a Chimeroid and the pair 
of a Hybodont indicates not only an affinity between the orders 
to which they belong, but affords, I believe, a clue to the ex- 
planation of the disparity of size between the two spines of the 
latter; a disparity greater than obtains in any existing Sharks 
possessing such defences. Agassiz himself inferred a relation- 
ship between the Sharks and Chimera from dental characters, 
Cochliodus being one of the links (and Cestracion surely 
another?) which indicated the connection of forms whose 
teeth are otherwise so totally dissimilar in every character. 
Charlesworth, in 1839,{ suggested ‘an apparent analogy’ be- 
tween the single ‘ frontal spine’ of Hybodus and the peculiar 
apparatus upon the head of the male Chimera; and although 
Hybodus has, as we now know, at least four of these cephalic 
spines,§ yet I believe that Charlesworth’s suggestion is a good 
one. An examination of aremarkably fine slab of Chimeroid 
remains from Solenhofen, now in the British Museum, showed 
me that the numerous hooklets, terminating the large cephalic 
spine there preserved, have a marked resemblance on a small 
scale to the enamelled portion of the ‘ Sphenonchi,|| and it 
would thence appear that the one large bony support may be 
analogous to the four tricuspid bases. Considering the wide 
difference that there must be between Hybodus and Chimera, 
we should not expect that the resemblance in structure of 
*Is not the spine which Agassiz called Leptacanthus, and classed with the 
Hybodonts, that of a Chimeroid? At least, it has only occurred in strata in 
which Chimeroid jaws have likewise been met with. 
t Ag. ‘ Pois. Fos.’ vol. iii. p. 336. { Mag. Nat. Hist. 2nd ser. vol. ii. p. 246. 
§ See note by Miss Anning, ibid. p. 605, and which statement I myself have 
been frequently able to verify. 
|| The name given by Agassiz to these fossils, which he considered to be the 
teeth of a Hybodont. 
