Mr. T. Stock on the Genus Tristychius. 181 
hemal aspect*. The spines of the neural row where best 
preserved are about 4 lines in height, triangular, and acutely 
pointed, the points being directed backwards. The hemal 
spines (not well preserved) appear to alternate with the 
neural, are apparently of the same size and shape, and are 
directed forwards. Some allowance must be made, however, 
in this description, for post mortem disturbance and alteration. 
The axis evidently consisted of a persistent notochord. 
In this (so far as I am aware) the earliest known shark in 
which readable traces of the axial skeleton have been pre- 
served, it is important to observe that these conform in rather 
a significant way to those types of vertebral organization which 
are regarded as the most ancient and the simplest. The dis- 
covery of this single specimen, however, is not sufficient to 
justify any certain conclusions as to the presence or absence 
of a more highly organized skeleton in other sharks of the 
same age. Prof. Newberry has indeed drawn attention { to 
an interesting specimen found in the Carboniferous rocks of 
Ohio (Waverly group), which he considers to represent a grade 
of organization in some respects higher than in most of the 
sharksof the present day. His remarks are of great interest, and 
need no excuse for being quoted. He says :—‘ I should also 
mention in this connexion a remarkable shark’s tail found at 
Vanesburg, Kentucky. . . . This specimen, which is nearly 
a foot and a half long, shows the outline of the heterocercal 
tail of a shark which must have been 8 or 10 feet in length. 
The vertebral column is seen to reach far into the upper lobe 
of the tail, The vertebre have certainly disappeared, leaving 
a smooth band to mark the space they occupied. This is 
bordered on either side by the impression of linear pointed 
apophysial bones, which were evidently much better ossified 
than the centra of the vertebra. The lower lobe of the tail is 
formed by a number of strong ossified rays! ‘This shows that 
this Carboniferous shark. ... . had a skeleton in some re- 
spects more fully ossified than most of the sharks of the 
present day.” This brief notice, which he promised to 
supplement by a full description {, possibly may not represent 
his riper views as to the reading of the specimen; but it 
appears to me that, taking the description as it stands, there 
are several assumptions which are scarcely warranted by the 
facts; and chief of these is the implication that the axis was 
* I consider the upper row as figured to be neural, though there is not 
much to show which is neural and which is hemal. 
+ Geol. Surv. Ohio, vol. i. part 2 (Paleontology), p. 279. 
{ I am quite ignorant whether his promise has been fulfilled or not. 
