184 Mr. T. Stock on the Genus Tristychius. 
Ag. This is a matter of evident importance; for upon the 
correctness of the reference depends a great part of the value 
of this communication. I have therefore given figures (Pl. VII. 
fies. 13 & 13 a) of the best-preserved fragment, and placed 
them beside the typical form (Pl. VII. fig. 7) for comparison. 
The number of spines found with this specimen is quite 
conclusive to my mind as to the fact that some of them (per- 
haps all) belonged to the horizontal fins. If any were dorsal 
(prepinnate or not), they did not differ from the paired 
spines. Yet a real distinction has apparently been established 
by Messrs. Hancock and Atthey* between the paired and 
dorsal spines of Gyracanthus ; and, from analogy, we should 
be prepared to expect that that distinction would hold good 
for other genera that possessed both. 
There is no evidence of the presence of sphenonchi, the 
cephalic spines found associated with Hybodus. 
Cuticular appendages.—There are a number of minute 
bodies scattered all over the stone, which are the remains of 
the dermal skeleton. They are so small, however, so much 
fractured, and simulate by their sculpture (where obscurely 
seen) the grooved cusps or bases of the teeth so closely, that 
it is exceedingly difficult to get a view of a specimen suffici- 
ently isolated to be able to say with certainty what was its 
shape. In front of the largest fragmentary spine is a cluster 
of them, where they are rather better seen than elsewhere. 
They bear a pretty strong resemblance to clusters of caraway 
seeds. Sometimes two and sometimes three appear to be placed 
together, with distinct interspaces, however, and each traversed 
by one or two rather coarse stria. The difficulties of ob- 
servation, however, are so great that better material may correct 
this description. On holding the slab obliquely to the light 
and looking carefully with the lens, I think any one would, 
at any rate, be convinced of the existence of these tubercles, 
though opinions might differ as to their configuration. One 
that I thought I had isolated proved on more careful observa- 
tion to be the base of a broken tooth, the strize only distinctly 
appearing above the matrix! Sometimes I have thought 
that they approach the body figured by Giebelt as belonging 
to Hybodus carbonarius, and which there is good reason 
for believing was an appendage of the dermis. I have tried 
to figure a specimen, Pl. VII. figs. 15 & 15 a, which figures 
present my view of its shape; but I must leave to the future 
* Northumb. & Durham Nat. Hist. Trans. vol. iii. p. 109. 
+ Loc, cit, (reproduced Pl. VII. figs. 14, 14a), His figure is probably 
incorrect, 
+ 
