62 Day—On Acrodus. 
character. That of Hybodus differs, however, from that of 
Acrodus Anningie in being gently and regularly curved off 
from the sides to the centre of its posterior face, instead of this 
forming almost a right angle with the side; the ridges of the 
former are more numerous than in the latter; and a still better 
distinction is found in the largeness and fewness of the denticles 
upon that of Acrodus. Of the posterior spine (PI. LV. fig 7) about 
an inch of the base is missing; the length of the spine, as it 1s, is 
about 114 inches, of which ale fragment of the base measures 
24 inches. The exposed part of “the spine would thus have 
measured 9 inches, or 44 less than what is preserved of its 
anterior fellow. From the latter it differs also in its much 
stouter proportions; and it has moreover a slight angle at the 
upper part of the base, which gives a somewhat distorted 
appearance to the spine, and the effect of which doubtless was 
to give to the exserted portion a more backward inclination 
than that of the anterior spine; the inserted portions probably 
penetrating the body in parallel directions. The line formed 
by the termination of the polished ridges, at the upper edge of 
the base, in the smaller spine is not so sharply sloped from the 
posterior to the anterior edge, but takes a deeper curve than 
in the larger. In the squareness of the posterior side, and in 
the comparatively large size of the denticles, the two spines 
resemble each other. 
After a careful comparison of several spines more or less 
closely resembling the one described with the figure of Hybodus 
curtus given by Agassiz, I am strongly of opinion that that is 
nothing more than the posterior dorsal spine of <Acrodus 
Anningie; whilst the anterior spine is generally marked in 
collections as Hybodus grossispinus. 
Since Agassiz published his work certain spines, obtained at 
Lyme Regis, have been assigned to Acrodus, having been 
found associated with teeth of the genus. One of these in my 
possession, and which appears to be an anterior, differs from 
those described in being almost smooth, the elevations being 
nearly lost, though the polished markings are preserved ; in its 
shorter and stouter proportions, and moreover in being scarcely, 
if at all, curved. Such spines probably belonged to A. nobilis 
or to A. latus; but the differences between them and those of 
A, Anningie, or of undoubted Hybodonts, are clearly only 
differences of degree. 
The fact that these two genera are more closely allied than 
supposed by Agassiz has long been admitted by many,* though 
* Charlesworth, Mag. of Nat. Hist. 2nd ser. 1839, p. 245. Owen, ‘Odontography,’ 
as quoted above by Agassiz. Ibid. ‘ Palsontology,’ p. 108. Mackie, The Geologist, 
