EXPLAXATION OF CATALOGUE. XI. 



expiratiou, which is generally in the form of a seiiiilunar fissure, convex 

 backwards. They are 2)rotectcd by a cover of bony plates, called the 

 02}ercuhim the posterior margin of which is free. Beneath this is a thin 

 membrane (gill-membrane), which is folded, and supjioi-ted hy bony rays, 

 called hranchiostegals. The gills in most fishes consist of a series of small 

 triangular leaflets of equal breadth {pectinated or laminated gills), 

 attached by their bases to the branchial arches. These leaflets are nearly 

 always in two rows on each branchial arch, but if only one such row 

 alone be present {e.g. Trachelochismus), then the name of half gill is 

 given to such an arch. In the bony fishes the branchial arches are free 

 on the outside, but in the Sharks and Rays there proceeds from each 

 bi'anchial arch, between the branchial leaflets of the anterior and posterior 

 row, a membranous prodiiction as far as the skin, which entirely covers 

 the gills with the exception of abovit five apertures (gill oj^enings) for 

 the expulsion of the water. Many fishes have false gills {lyseudo 

 branchiae) on the palate, above and outwards from the gills ; these must 

 not be confounded with the half gills. 



" Head. — Below the orbits of the eye is placed a row of bony scales, 

 forming a half ring {siihorUtal ring), the anterior of which forms the 

 outer or inferior mai-gin of the nasal cavity. The upper jaw bones 

 (maadllaries) are commonly without teeth, and lie in the upper lip ; 

 they run obliquely backwards and downwards without being connected 

 at the extremity with other bones. The intermaxillary bones form the 

 central part of the mouth, and are usually armed with teeth, as are also 

 the palatine bones, which are situated on each side of the vomer or 

 central part of the roof of the mouth. Behind each of these palatine 

 bones lie two bones called the pterygoids. The lower jaw is divided into 

 a right and left portion {mandibuloi), united in the middle (symphysis) 

 by cartilage. The tongue bone lies under the head, and usually consists 

 of a series of unjiaired bony pieces placed behind one another. To these 

 bony jiieces five arches are attached, lilce ribs to a sternum ; the 

 hindermost arch is incomplete and Ls simply laid on the under side of 

 the gullet j it consists of two bones (inferior 2)haryngeal bones), one on 

 each side, which are sometimes united to form a single unpaii-ed bone. 

 (Order II.) 



" Teeth. — The teeth present much variety, and in some kinds are 

 entirely absent. In othei's they exist in great numbers in the jaws, and 

 on the vomer, palatine bones, tongue, branchial ai-ches, and pharyngeal 

 bones. [In form also they difler very much. Sometimes they are small, 



