GENERAL STRUCTURE. 
3*9 
It must not be supposed, however, that even the fringed type is the most 
primitive form of fin known, since in an extinct armoured shark ( Cladoselache ) 
from the Carboniferous formation, we meet with what may be called the fold-type 
of paired fins. In these fishes (one of which is figured in the sequel) the pectoral 
and pelvic fins are placed far apart, but in the same longitudinal line, and are 
formed of a series of parallel cartilaginous rods arising from an extended base, 
and projecting at right angles to the body; the pectoral pair being considerably 
the larger. Assuming that fishes originally possessed on each side of the body a 
continuous fold of skin, strengthened by parallel cartilaginous rods projecting at 
right angles, this fold-type is exactly what we should expect to find in the 
evolution of pectoral and pelvic fins, by the disappear¬ 
ance of a considerable portion of the original fold, and 
the development and basal contraction of the remaining 
moieties. 
Although the structure of the skull is of the 
highest importance in the classification of fishes, our 
remarks on this subject must be very brief. In the 
skulls of the higher bony fishes the original cartilaginous 
cranium, which persists in the sharks, is overlain by a 
number of membrane-bones, the names and position of 
which are indicated in the figure of the perch’s skeleton 
on p. 316. Among these the pterotic and sphenotic are 
peculiar to the class, and there is always a large para- 
sphenoid underlying the base of the skull. The inter¬ 
vention of the elements known as the liyomandibular 
and symplectic between the squamosal and the quadrate is 
unknown elsewhere, although it is by no means universal 
among fishes. Among other bones may be mentioned the infraorbital ring 
beneath the eye; as well as the preopercular, subopercular, interopercular, and 
opercular, collectively constituting the gill-cover or operculum of the bony fishes. 
In the gill (or branchiostegal ) membrane, which joins with the gill-cover in closing 
in the gill-chamber, there may be developed a number of gill (or branchiostegal) 
rays ; but these may be partially or entirely replaced by jugular plates, occupying 
the space between the two branches of the lower jaw. It will be unnecessary to 
mention by name the various bones constituting the hyoid arch, which is attached 
to the inner side of the liyomandibular by the stylohyal, and extends forwards to 
support the tongue; and it will suffice to state that behind this arch are 
situated the branchial or gill-arches, to the inner margins of which are attached 
the spine-like gill-rakers. In the lower jaw, or mandible, there is usually both a 
dentary and an articular piece; but an angular, and more rarely a splenial or 
coronoid element may likewise be developed. 
The teeth of fishes present a greater degree of variation than 
is found among any other class of Vertebrates. While in some 
cases they may be totally wanting, in others they may be developed on all the 
bones of the mouth, and even on the hyoid bones and gill-arches; and they may 
be attached only to the membrane lining the cavity of the mouth. Frequently 
SKELETON OF PECTOEAL FIN OF 
AN EXTINCT FRINGE-FINNED 
shark. (From Fritsch.) 
