PLECTOGNATHI. 



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Fig;. I4.S. — Balistes ^eograiih 



Aluteres, have the body 1011";, the granulations 

 scarcely visible, and a single spine in the first 

 dorsal, but the pelvis is completely hidden in the 

 skin. 



Triacanthus, 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 



vertebra" are also soldered together. The jaws are furnished with a row of ten or twelve conical teeth ; and tliey 

 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 difterences 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 

 be considered either superior or inferior to the Ordinary Fishes ; for, while some of the 

 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 

 as the Marsupial Mammalia range parallel vi^ith 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 

 piece without sutures ; but there are ridges, furrows, and holes, whereby the por- 

 tions of it analogous to the cranial bones of other fishes may be distinguished. Even the 

 moveable articulations of other orders are not distinguishable in the whole of this : as, 

 for instance, part of the vertebrae of some of the rays make a single piece, and some 

 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. 



