Mr. T. Stock on the Genus Tristychius. 189 
cock and Atthey for breaking down the traditionary notion 
that all fossil spines were dorsal. 
In a clever research* (based upon very scattered material) - 
they showed that spines of G'yracanthus, till then believed to 
be dorsal, were really pectoral ; and I have been able to con- 
firm their conclusionsf by the finding of an interesting speci- 
men containing well-preserved remains of the pectoral arch. 
I consider that Messrs. Hancock and Atthey, by these obser- 
vations, have opened up a very promising future for selacho- 
logical inquiry ; for whilst the soft structures of the fins will 
(except in the rarest cases) have left no trace of their exis- 
tence, we may expect that the spines which protected them 
will frequently be preserved in their proper relations. 
I have thus tried to hint at the lines upon which any real 
advance in our knowledge of the Paleeozoic sharks will pro- 
bably be made. [am unfortunately not able to utilize them 
to any great extent in the discussion of the affinities of T7isty- 
chius. By the spines it is allied to Ctenacanthus through 
Tristychius minor, Portlock. 
The appendages of the cutis differ greatly from those found 
on Hybodust. They bear, however, a strong resemblance 
to those of Otenacanthus hybodoides, Ag. (= Cladodus mira- 
bilis, Ag.§), and Gyracanthus tuberculatus, Ag. Their 
value for purposes of classification, however, is probably slight, 
judging from recent genera. 
The teeth of Tristychius are even more Hybodont in facies 
than the probably closely allied teeth which Giebel referred to 
Hybodus |\. 
The Ctenoptychius pectinatus tooth (if proved to belong to this 
genus) would detract a good deal from the (probably extreme) 
value which has been attributed to the amount of the lateral 
cuspidal elevation of the teeth, in defining species and genera 
commonly ascribed to the Hybodontide. It is extremely 
unfortunate that the evidence is no clearer either for or against 
this unexpected association. 
After weighing the evidence now set forth, I incline to the 
belief that Zristychius was Hybodont, but not Hybodus, and 
should not be much surprised if the future proved that Clado- 
dus (or a part of it) was a closer ally than Hybodus. 
* Loc. cit. 
+ Ina paper read to the Edinburgh Naturalists’ Field Club, but not 
yet published. 
{ I have had few opportunities of studying Hybodus. Excellent unde- 
scribed Mesozoic material exists; and it 1s much to be desired that some 
one who has access to it would describe it. 
§ Messrs. Hancock and Atthey have brought forward an amount of 
evidence which amounts to proof that the tooth of Ctenacanthus hybo- 
doides, Ag., was Cladodus mirabilis, Ag. || Loe. cit, 
