On the Fossil Fishes of the Carboniferous Limestone Series of Great Britan. 529 
Armagh limestone, which have external ornamentation more or less characteristic 
of Oracanthus. They are of varied form, but all agree in being thin osseous plates, 
whose surface is raised so as to form smooth enamelled pustulations similar to, but 
smaller than, those on the surface ef the Armagh specimens named by M. Agassiz, 
Oracanthus confluens. Amongst this group must be included the examples named 
by Prof. Moy, Coccosteus carbonarius, Asterolepis verrucosa, and Platycanthus 
isosceles. That this group of specimens formed the cephalic covering of some fish 
there can be little doubt. Figures 1 and 2 probably formed the central dorsal 
shield ; figs. 4 and 7, the maxilla and mandible respectively ; figs. 5 and 6, the 
operculum or cheek plates ; fig. 12 has the form of a jugular plate, and fig. 13, 
an external bony plate, bent at right angles. The upper triangular portions of 
the plate represented in fig. 2, broken or separated from the gencral mass, have 
been labelled O. minor, and what appear to be similar fragments have been con- 
sidered by American Paleontologists as representing a separate genus, Pnigacanthus. 
The specimens so named in the collection of Lord Enniskillen are quite different 
from the type of the species described and figured by Prof. Agassiz; the latter as 
already observed, does not possess the characters of the genus. In addition to 
those specimens which are represented on Plate LX. there is a large number 
worthy of very careful study, for the most part similar in form and characters to 
those figured, but frequently differmg in detail, especially as to form. 
Three specimens, from the extreme peculiarity of their form, call for especial 
attention. They are apparently similar to each other and may have been the 
counterpart organs of three individuals. The largest and most perfect specimen 
is 14 inches in length, the basal termination absent. The specimen consists of a 
lone shaft, compressed from back to front, slightly arched longitudinally. Its 
greatest lateral diameter 1s about an inch towards the basal extremity, antero- 
posteriorly it is between °25 and ‘5 of an inch. Nearer the upper extremity of 
the specimen the diameter slightly and gradually decreases until about 2 inches 
from the extremity where it suddenly expands to 1°75 of an inch, and forms a 
more or less spatulate expansion, ending with a broadly rounded margin, The 
specimen is composed of a close-grained, fibrous, osseous substance ; one of the 
specimens shows an internal cavity near the basal end, it is small and conforms 
generally to its outer surface ; the spatulate extremity appears to be a solid bony 
structure. The outer surface is convex, and ornamented with pustulate tubercles, 
sometimes indiscriminately scattered over the surface, and at others a short 
distance apart, assuming a more or less definite arrangement in gyrating lines ; 
the tubercles appear quite similar to those of the external covering of other parts 
of the fish, that is, assuming that the dermal plates already mentioned belong to 
the same species. The inner surface of the bone is smooth or striated longitud- 
inally with a concavity corresponding to the outer convexity. The lateral portions 
formed by the contact of the anterior and posterior surfaces are acutely angular. 
412 
