Mr. T. Stock on the Genus Tristychius. 185 
the corroboration or disproof of the accuracy of the repre- 
sentation. If my reading is correct, they bear a rather stri- 
king general resemblance to those found associated with the 
spines of Ctenacanthus hybodoides, Ag., for the knowledge 
of which association we are indebted to the painstaking re- 
searches of Messrs. Hancock and Atthey. I owe a large 
fragment of this spine to the kindness of my friend Mr. J. M. 
Campbell, of the Kelvin Grove Museum, Glasgow, who ob- 
tained it from the Coal-measures of his district. On splitting 
the shale on its posterior side I found, as I was led to expect 
I should probably find from my experience with Gyracanthus*, 
several well-preserved tubercles, of one of which I give a 
figure (Pl. VII. fig. 16). It has never, so far as I remember, 
been figured before ; and a comparison of it with the tubercle 
of Tristychius arcuatus, Ag., will at once show that there is a 
decided resemblance between the two. A strong resemblance 
exists also between them and the tubercle of Gyracanthus 
(Pl. VII. figs. 17 & 17a); and these are all, or nearly 
all, of the Carboniferous sharks upon which dermal appen- 
dages have as yet been clearly provedy to exist. Nevertheless 
this resemblance may not necessarily imply close affinity. 
Cranial Cartilage.—TVhe cranial cartilage preserved in this 
specimen presents the usual mosaic appearance with which 
the student of fossil sharks is familiar. When freshly frac- 
tured, the black glossy appearance, due to the change to 
bitumen, is apt to deceive the unwary by an appearance 
simulating that of minute ganoid scales, such as are found on 
Acanthodes. 
The next and last specimen to be described occurs in a 
nodule, found by myself at Trinity near Edinburgh, in the 
Calciferous-Sandstone series (Wardie horizon). 
The nodule is not entire ; it contains, however, the remains 
of the cranium, and is valuable as throwing light upon the 
dentition of this fish. The nodule measured 5x4 2 in. 
(before it was broken up tor examination) and was very 
pyritous and hard. 
Description.—I succeeded in developing a fragment of one 
spine. It conforms in sculpture so closely to those last de- 
scribed that all might have belonged to the same fish. What 
can be seen of its shape agrees also ; and the denticulation on 
the posterior area is identical. There can be no doubt that 
it belongs to the same species. 
* The shale at the back of the spines should always be carefully ex- 
amined for tubercles. i 
Tt Many very different dermal buckles or tubercles, however, haye re- 
ceived special names, having been mistaken for teeth. 
