FROM LYME REGIS. Q47 
er stvie; from Hyb. reticulatus they are at once distinguished 
by the bluntness of the dental processes, and the divergence 
of the strie upon their surface’: the teeth of Hyd. medius 
differ in the indistinctness of the diyision of their crowns into 
lobes, and in the regularity of the compressed ‘cones which 
their forms exhibit. j 
By far the greater number of species of Hybodus enume- 
rated in the ‘Poissons Fossils” are merely known by the Jch- 
thyodorulites; and Agassiz regrets that the rare occurrence 
of the teeth and spines under circumstances which establish 
their relation’ to the same individual, compels him to intro- 
duce a double nomenclature. It fortunately happens in the 
present instance, that in addition to the frontal spine, both ~ 
dorsal were also discovered. ‘One of these is represented in 
the plate of the natural size, (fig. 9). Its fellow differs in 
being about two inches longer, in presenting a much slighter 
decrease in the width of the grooved sides, and in having a 
smaller number of denticulations. The character of these parts 
is so well conveyed by Mr. Sowerby’s engraving, that it is 
unnecessary for me to enter into any minute description. 
Inthe future identification of this species, it should be 
borne in mind that the supposed’ frontal spine may be lost, 
while the jaws or other parts of the skeleton may be pre- 
served; and on the other hand, this singular appendage may 
not improbably be common to several species of the genus. 
The circumstance of this fossil appearing to be generally un- 
known ‘to collectors, even in a detached state, strengthens my 
assumption that the shark to which it belonged’ was not fur- 
nished with a series of these spines, and indeed perhaps in- 
dicates the rare occurrence of the particular species which it 
may serve to characterise. I hope. the publication of this 
notice may draw the attention of those interested in fossil 
Zoology to’ the subject, but at present Mr. Lonsdale is the 
only person of those to whom the fossil in question has been 
shown, who had previously seen anything of the kind; and 
he tells me that a similar but detached fossil body, also from 
the lias, is preserved in the Bath Museum. 
Being unable to identify the present species with any one 
that has been described, I have called it Hybodus Delabechett, 
M. Dela Beche having, Ibelieve, been the first geologist who 
drew attention to the fossil remains of the genus. 
REFERENCES TO PLATE NO. 4. 
Figs..1 & 2i:: Tabular masses upon the borders of which the teeth are dis- 
posed. -The posteridr‘and: right lateral: borders’ extend) farther than! re- 
