VURTEBEATA. 187 



Order V. PLECTOGNATHI. 



SCLERODERMI. GYMNODONTES. 



Skeleton only partially osseous. Gills pectinate ; a narrow 

 gill-aperture on each side. No true scales, but either bony plates 

 firmly united or a spiny skin, or naked. 



The body is either covered by large bony plates united together 

 to form an inflexible case, as in Ostraciontidos, with small scale- 

 like movable plates as in Balistidae, with a rough granulated or 

 spiny skin as in Gymnodontidas, or naked as in Orthagoriscus. 

 The maxillary and intermaxillary bones are united, forming the 

 jaw, which is immovable. The gill-covers are hidden beneath 

 the skin. 



The Globe-fishes, a few of which are found in fresh water, 

 blow themselves up by swallowing air, which is retained 

 in a dilatation of the oesophagus. They have as well an air- 

 bladder. Their teeth are represented by the ivory-clad termina- 

 tion of their jaws. They form, with Orthagoriscus, the family 

 Gyrunodontes of Cuvier. His Sclerodermes comprise Ostracion- 

 tidas and Balistidae. 



Ostraciontid(S. Monacanthus. Diodon. 



Ostracion (Trunk-fish). Triacanthus. Triodon. 



BalistidcB (File-fish). Gymnodontida Orthagoriscida. 



Balistes. (Globe-fish). Orthagoriscus=Mola 



Tetrodon. (Sun-fish). 



Order VI. CHONDKOPTERYGIL 



NANTES. CARTILAGINEI. PLAGIOSTOMI. PLACOIDEI. 

 ELASMOBRANCHII. SELACIIII. 



Skeleton cartilaginous ; skull without sutures. Gills fixed, 

 communicating externally by five to seven slit-like apertures. No 

 gill-cover, or rudimentary. Bulbus arteriosus rhythmically 

 contractile. 



The mouth is transverse, placed beneath the head. The ribs 

 are small or rudimentery. The tail is unsymmetrical (hetero- 

 cercal). The skin is composed of small portions of dentine in the 

 form of granules, tubercles, or spines. There is no air-bladder ; 

 and the optic nerve is not decussating. In the males are two 



