LAMPREYS AND FISHES OF INDIANA. 153 



in the base of the upper barbel. In front of the eye is found a fiat bone, 

 the preorbital, while a chain of suborbitals runs below the eye. In some 

 fishes, as in the miller's thumb, these are converted into a firm bony stay 

 underneath the eye. 



The lower jaw is composed of a number of pieces, the most important 

 of which is the dentary, so called because it usually supports teeth. In 

 the roof of the mouth are several bones, important because they are often 

 armed with teeth. In the middle line, just behind the premaxillaries, is 

 the vomer. The teeth on the bones in the mouth can most easily be de- 

 tected by gently scraping over them the head of a common pin. Im- 

 mediately behind the vomer, on each side of the roof of the mouth, is the 

 palatine, and further back the pterygoid. The palatines often bear teeth, 

 as in the ];)ike, the bass, etc. Teeth on the pterygoids are not so com- 

 mon, but they may be found in the goggle-eye and the warmouth. 

 These teeth will serve to locate the positions of these bones. 



The sides of the skull behind are occupied by the opercles or gill-covers, 

 a sort of valves, which cover in the gill-chamber. The gill-cover almost 

 always consists of four bones, the preopercidum, opercidum, interopercidum 

 and the suboperculum. The free border of the preoj^ercle is often furn- 

 ished with teeth, serrated, while the opercle may bear one or more spines. 

 The yellow perch has both the serrations and an opercular spine. The 

 gill-covering has a membranous border by means of which the gill-cavity 

 may be more completely closed. Below the oper(3ular apparatus is found 

 the gill-membrane, a fold of skin supported by one or more ray-like bones, 

 the branchiostegals. The anterior ends of these bones are attached to the 

 hyoid bone. The gill-membranes may be joined along their lower borders 

 to the isthmus, the space between the throat and the breast ; or they may 

 be free from the isthmus and from each other ; or lastly, they may be 

 free from the isthmus and grown together, so as to form a sort of flaj) — 

 like a bib. 



The gills of fishes consist of folds and slender processes of the mucous 

 membrane, supported by the cartilaginous or bony gill-arches. These 

 arches, four in number, consist each of several pieces of bone or carti- 

 lage, and the two of each pair encircle the pharynx. The mucous mem- 

 brane forming the gills is produced into flat processes, which are arranged 

 in one or two rows along the convex side of each arch. More commonly 

 there are two rows, but in some cases only a single row of the gill-fila- 

 ments on each arch. Along the concave side of the arches are often 

 borne solid processes of bone or cartilage, the gill-rakers. These are 

 sometimes short and weak, sometimes thick and stout, sometimes long 

 and slender. In the paddle-fish and some others they are found on all 

 the gill-arches ; they are long and slender, and evidently act as a filter- 

 ing apparatus. Commonly only the most anterior arch bears gill-rakers, 

 as may be seen in the bass and some of the sun-fishes. The remains of a 



