530 On the Fossil Fishes of the Carboniferous Limestone Scries of Great Britain. 
The outer or anterior portion of the spatulate extremity is much worn, apparently 
by abrasion during the lite of the fish (see figs. 3, 4, on Plate LXV.) 
Along with the numerous examples of the external covering alluded to above, 
there is in the Armagh limestone a number of ichthyolithes, which were considered 
by Prof. Agassiz and others, as the spines of Orthacanthus confluens. On Plate LXIIT., 
figures 1, 2, 3, three of these specimens are represented so as to indicate some of 
their peculiarities. They are laterally much compressed, more or less triangular in 
outline, with one side longer than the others, the junction of the two shorter forming 
aright angle. It will also be noted that the specimens are in some cases dextral, 
in others sinistral, and it may be inferred from this that they were probably 
associated in pairs. The outer surface is covered with considerable uniformity by 
amore or less irregularly distributed series of pustulate tubercles, smooth and 
apparently worn at the apex, with radiating ridges descending therefrom, and 
absorbed in the interspaces forming the general mass of the bone. The tubercles 
extend across the surface parallel with the open or basal extremity, seeming to 
indicate successive lines of growth. When these rows of tubercles are followed 
from the front to the back of the specimen it is observed that they are continuous 
and extend evenly and around its surface, but that on the lateral surface the rows 
are rapidly diverted towards the pointed apex, whilst on the posterior surface the 
lines of tubercles arch stiJ] more in the direction of the point. This characteristic 
is well shown in Plate LXIII.,figs.1,1a. The basal orifice, which is extremely large, 
conforms to the outline indicated by the rows of tubercles, the anterior or outer 
surface extending much lower than the posterior or inner one. The walls are very 
thin and the internal cavity extends nearly to the point. 
As already stated these cone-like ichthyolithes have existed in pairs, and it appears 
probable that they formed the posterior termination of the body. From the 
extension of the hard external covering on one side to a much larger extent than 
the other, it may be inferred that those sides with least covering were in juxta- 
position and formed the axis of the body from which the two armour-plated 
prolongations were expanded. Should this have been the arrangement, the vertical 
thickness of this portion of the fish appears to have been considerably greater than 
the lateral. 
It is not proposed to treat more fully of this part of the subject at present, but 
rather to leave for future opportunity, with the hope of increased material, the 
elucidation of what now appears to be a somewhat difficult problem in fossil 
ichthyology. All the specimens hitherto mentioned are from the Mountain 
Limestone of Armagh, and are in the collection of the Earl of Enniskillen. 
The specimens of Oracanthus from the Mountain Limestone of Bristol, are longer 
in proportion to their diameter across the base than those of Armagh, the average 
length being double the diameter, whilst in those from Armagh the length is about 
equal to the greatest breadth. In other respects they are similar. A specimen, 
