188 Mr. T. Stock on the Genus Tristychius. 
pear to have been of two kinds :—one prehensile, with well- 
developed median and lateral cusps ; the other without specially 
prominent elevations, and occupying a different position in the 
mouth. The skin was clothed with a dense armour of tuber- 
cles, the veritable shagreen of these ancient sharks. The 
horizontal fins (or some of them) were protected anteriorly by 
gracefully curved, ridged, grooved, and denticulated spines*. 
(The presence of dorsal spines, protecting fins or otherwise, is 
not proved.) There were at least four on every fish. They 
varied a good deal in different individuals, but were ddente- 
cally similar in the same fish. The peculiarly curved spines 
known as sphenonchi and found with Hybodus do not appear 
on this fish. The endoskeleton was cartilaginous. ‘The 
axis was persistently notochordal and unsegmented, but gave 
off (neurally and hemally) spines that were composed of 
granular cartilage, closely apposed, probably alternately ar- 
ranged on opposite sides of the axis, and directed back- 
wards, the whole conforming to the simplest and most 
ancient types of axial structure. ‘These statements, or the 
majority of them, rest upon tolerably well-ascertained facts, 
and though far from sufficient to elucidate the whole structure 
of the fish, form at any rate a useful contribution to its 
history. 
A ffinities—Certain characters of which much is properly 
made in the classification of recent Selachian fishes, such as 
the presence or absence of the membrana nictitans, the conflu- 
ence of the nostrils with the mouth, the presence or absence of 
spiracles, and the notching of the pectoral fins at their origin, 
are of course practically inapplicable to most sharks in a 
fossil condition, notwithstanding the surprising perfection in 
which such remains (in post-Paleeozoic rocks) have been ob- 
tained. Nor do I think that the important series of investi- 
gations now being carried on with so much zeal by Prof. 
Hasse} will be of much service to students of the Selachian 
remains of the older rocksf. 
There abides, however, a valuable set of characters by the 
use of which a natural classification will no doubt be gradually 
conquered. Important amongst these are the pinnation and 
dentition. Science owes a debt of gratitude to Messrs. Han- 
* T have seen one or two spines considerably larger than the largest 
figured. 
+ Das natiirliche Syst. d. Elasmobranchier. 
{ He arrives at a singular conclusion with regard to Tristychius. He 
says (op.cit. 1st part, p. 62), “ Alle diese Formen (Asteracanthus, Myria- 
canthus, Priscacanthus, Tristychius) smd demnach meiner Ansicht nach 
jiingeren Holocephalen zuzurechnen, welche sich mit Asteracanthus bis 
in den mittleren Jura hineinerstreckten und von denen Yrestychius in 
seiner Form sicham meisten an unsere jetzt lebende Chimera anschloss.” 
