PLECTOGNATHI. 
329 
Aluteres, have the body long, the granulations 
scarcely visible, and a single spine in the first 
dorsal, but the pelvis is completely hidden in the 
skin. 
TriacantMis, has a kind of ventrals, each sup- 
ported by one large spinous ray, adhering to a non- 
projecting pelvis ; the first dorsal has one largish 
spine, and three smaller ones behind it ; the body 
is crowded with small scales ; and the tail is longer 
than in any of the other subgenera. The single 
known species inhabits the Indian Ocean. 
Ostracion, the Trunk-fish, has the head and body 
covered in such a manner with plates of bones, 
soldered together, as to form an inflexible cuirass, 
leaving only the tail, the fins, the mouth, and a 
small margin of the gill-opening, capable of mo- 
tion,— all of which moveable parts pass through 
openings of the cuirass. The greater part of the 
vertebrae are also soldered together. The jaws are furnished with a row of ten or twelve conical teeth ; and they 
have no apparent gill-opening, except a mere slit with a cutaneous lobe ; but inside the skin they have a gill-lid and 
six rays. They have neither pelvic bone nor ventrals, and the single dorsal and anal are both small ; they have 
little flesh, but the liver is large, and abounds in oil ; the stomach is also very large and membranous. Some of 
them are thought to be poisonous. They might be subdivided according to the form of the body and the spines, 
but it is not yet ascertained whether there may not be sexual dilferences in these respects. [The body is triangular 
in some, quadrangular in others, and in some it is compressed ; and the appearance of the cuirass, or covering, 
varies still more. None has been met with on the British shores.] 
CHONDROPTERYGII. 
The second series of Fishes, the Chondropterygii, or Cartilaginous Fishes, cannot 
I be considered either superior or inferior to the Ordinary Fishes ; for, while some of the 
I genera resemble Reptiles in the structure of their ear and reproductive organs, other 
genera have the skeleton so very rudimental that one almost hesitates to regard them 
as vertebrated animals. They form a series, ranging parallel to the Bony Fishes, just 
j as the Marsupial Mammalia range parallel with the other ordinary Mammalia. 
Essentially, the skeleton is cartilaginous, — that is to say, it has no bony fibres, but 
! the calcareous matter is disposed in grains. The cranium is always formed of a single 
I piece without sutures ; but there are ridges, furrows, and holes, whereby the por- 
tions of it aiialogous to the cranial bones of other fishes may be distinguished. Even the 
j moveable articulations of other orders are not distinguishable in the whole of this : as, 
i for instance, part of the vertebr8e of some of the rays make a single piece, and some 
j articulations of the bones of the face also disappear. Among the latter, the most 
prominent character is the reduction of the maxillaries and intermaxillaries to mere 
rudiments concealed under the skin, while their functions are performed by the palatals, 
and sometimes by the vomer. The gelatinous substance which fills the intervals of the 
vertebrae in other fishes, and communicates from one to another by only a small hole, 
is, in several of this order, a long cord, which traverses all the vertebrae, with little 
variation of diameter. . 
The series divides itself into two orders : — Those with free gills, like all other 
Fishes ; and those with fixed gills, which are so attached to the skin by the internal 
edges that the water cannot escape from their intervals, except by holes in the surface. 
