44 
POPULAR SCIENCE REVIEW. 
throughout a considerable group of fishes, to which it is limited, 
it does not affect ichthyological researches in the same way as 
certain less profound alterations occurring in other types of 
fishes. It is for the knowledge of these that we are more 
especially indebted to the investigations of Dr. Gunther and 
Dr. Liitken. One of the commonest peculiarities of young 
fishes is the presence on the margins of the opercular bones, and 
especially of the prseoperculum, of numerous denticulations, 
which are gradually effaced by the growth of the bone, and are 
altogether wanting in the adult. In other cases certain bones 
belonging or in immediate contiguity to the head, such as the 
praeoperculum, supra- scapula, and humerus, become enormously 
developed, forming large plates, which regularly cuirass the 
front half of the fish or the greater part of it, and from these 
and other bones of the head formidable spines or other processes 
may be developed. Dr. Gunther figures a young Pomacanthus , 
in which the supra- scapular and prseopercular plates are thus 
produced into immense spines, the former concealing the fore- 
part of the dorsal fin, and the latter entirely hiding the ventrals. 
In this well-armed infant the frontal bone is produced over each 
eye into a long, pointed, lancet-shaped spine ; and altogether, 
small as it is, this young fish would probably be a very uncom- 
fortable morsel to swallow. In another similar form the frontal 
spines, instead of being straight and pointed, represent long, 
curved horns. These cuirassed young fish were formed into a 
distinct genus, named Tholichthys (in allusion probably to the 
dome-like bony front of the species first described) ; they are 
now recognized as constituting a stage in the development of 
certain fishes belonging especially to the Chsetodons and the 
allied groups Carangidae and Cyttidae. 
In many different forms strong spines are developed from 
various bones of the head and operculum, without any special 
dilatation of these into bony plates, as in the Tholichthys ; and, 
no doubt, in these cases also the protection of the young animals 
from the assaults of their enemies, is the office of these com- 
paratively formidable armatures. Such opercular and cephalic 
spines are characteristic of the young Swordfishes, in which 
they spring from the parietal and prseoperculum, and attain 
formidable dimensions ; and in these and other cases where the 
spines are greatly developed, they must, doubtless, be partially 
got rid of by absorption, or thrown off as the fish advances 
towards maturity. The development of the Swordfishes is 
particularly instructive, and we shall therefore endeavour to 
describe the stages through which they pass. 
The Swordfishes, the largest forms of the ordinary bony 
fishes ( Teleostei ), sometimes attaining a length of twelve or 
fifteen feet, constitute a family not far removed from the 
