220 YERTEBEATE ANIMALS. 



SuB-OEDER IV. Plectognathi. — Certain of the bones of the mouth (the 

 maxillary and prse-maxillary bones) immovably connected on each side of 

 the jaw. Integumentary skeleton in the form of ganoid plates, scales, or 

 spines. 



The chief families in this sub-order are the File-fishes (Balistidce), and the 

 Trunk-fishes ( Osiradontidce). 



Sub-order T. Lophobeanchii. — Gills arranged in little tufts on the 

 branchial arches. Integumentary skeleton in the form of ganoid scales. 



The two families contained in this division are the Sea-horses (Hippocam- 

 pidce), and the Pipe-fishes [Syngnathidoe). 



Oedek IV. Ganoidei (Gr. ganos, splendor, or brightness). 

 — The fourth order of fishes is that of the Ganoidei, including 

 few living forms, but having a great and varied development 

 in past geological epochs. The Ganoid fishes are dis- 

 tinguished by the imperfect development of the skeleton, 

 which is mostly cartilaginous throughout life, and by having 

 an integumentary skeleton composed of ganoid scales, plates, 

 or spines (Fig. 100, d). The skull is composed of distinct 

 bones, and there is always a lower jaw. There are usually 

 two pairs of fins (pectoral and ventral), supported by many 

 series of cartilages, and the ventral fins are placed very far 

 back. The first rays in the fins are usually in the form of 

 strong spines. The caudal fin or tail is mostly heterocercal 

 or unsyrametrical (Fig. 103, b). The swim-bladder is always 

 present, is often cellular, and is provided with an air-duct. 

 The gills and gill-covers are essentially the same as in the 

 bony fishes. The heart has one auricle and a ventricle ; and 

 the bulhus arteriosus is rhythmically contractile, is furnished 

 with a distinct coat of muscular fibres, and is furnished with 

 several transverse rows of valves. 



The best known of the living Ganoids are the Bony Pike 

 (Lepidosteus), the Sturgeon (Sturio), and the Polypterus. Of 

 these, the Bony Pike is found in the rivers and lakes of North 

 America. It is a large fish, attaining a length of several feet, 

 and it has the body entirely covered with an armor of ganoid 

 scales arranged in obliquely transverse rows. The jaws form 

 a long, narrow snout, armed with a double series of teeth, and 

 the tail is heterocercal. The vertebral column is more perfect- 

 ly ossified than in any other fish, the bodies of the vertebrae 

 being convex in front and concave behind ( " opisthocodous "). 

 The Polypterus (Fig. 107, A) inhabits the rivers Nile and 

 Senegal, and is remarkable for the peculiar structure of the 

 dorsal fin, which is broken up into a series of small, detached 

 portions, each composed of a single spine in front, with a soft 

 fin attached to it behind. Some of the species of Polypterus 



