TEEMS USED IN DESCRIPTION. 759 



with membrane bones (opprcula, etc.) in connection with it, and contain- 

 ing a braiu of several differentiated ganglia; shoulder girdle developed, 

 lyriform or furcula sh„pad; a distinct lower jaw; branchige, with the 

 outer edge free, attached to about five boay arches, which are connected 

 with the hyoid bone and parallel with the shoulder girdle, the hinder- 

 most pair modified into tooth-bearing "pharyngeals;" gill openings a 

 single cleft on each side behind the operculum, either confluent below 

 or else separated by an isthmus ; heart (typically) with two cavities and 

 an arterial bulb. In most fishes there is a membranous air bladder 

 lying immediately beneath the back bone, answering homologically to 

 the lungs of the higher vertebrates. In a few Ganoids the air bladder is 

 cellular and more or less functional, and connected by a glottis with the 

 oesophagus; in most of the soft-rayed species (Suckers, Minnows, Sal- 

 mon, Catfishes, etc.) there is a slender duct connecting the air bladder 

 with the alimentary canal; in the spinous fishes (Perch, etc.) this is 

 wanting. Reproduction by eggs of small size, which are usually fertil- 

 ized after exclusion ; the members of a few groups {Gyprinodontidm, Am- 

 blyopsidse, etc.) are ovoviviparous, the young being developed in a sort of 

 uterus. (Latin piscis, a fish.) 



Note. — The terms used in the description of fishes may perhaps best be made clear to 

 one not familiar with them by a sort of object lesson. The reader is supposed to have 

 at hand a epeoimen of the Common Brook Sucker (Oatostomus teres) and a Black Bass 

 (Mioropterua aalmoides). The general form of the body is, in th s memoir, usaally first 

 indicated in general terms, as elongate, oMong, short, deep, etc. ; more spscifia terms are 

 compressed (flattened laterally); (J«j)resse(i (flattened from above) ; /ifsi/ocm (spindle- 

 shaped, tapering each way from the shoulders); terete (nearly cylindrical, i c, the ver- 

 tical and horizontal diameters about equal. The depth of the body is described by com- 

 parison with the length, along the side from the tip of the snout to the base of the caudal 

 fin. The depth is measured at the deepest point, and is proportionately greater in old 

 fishes than in yonug. The general form of the head is nest noted ; then the form and 

 position of the mouth ; the mouth is terminal when'its opening is forwards, and the two 

 jaws are not very uaeqaal in length, as in the Blaok Baas ; it is inferior, when, by the 

 shortness of the lower jaw, its position is entirely underneath the snout, as in the 

 Sucker; it is oblique when its cleft slants backward and downward when the mouth 

 is closed; it is horizontal when the reverse is the case. The bones of which the jatv is 

 composed are the following : The two dentary bones joined by a symphysia in front, form- 

 ing the mandible or lower jaw; Vae premaxillaries, or inter-maxillariea, fortning altvays the 

 middle of the front part of the upper jaw, and in some cases forming its entire edge, as 

 is the case in the Black Bass. Attached to the pre-maxillaiies, eithar behind them, 

 as in the Blaok Bass, or below them, as in the case of the Trout, are the maxillariea or su- 

 pra-maxillaries. In the Black Bass these are conspicuous and flat, extending back below 

 the eye; in the fiucker they are hardly recognizable without dissection; in some fishes 

 there is attached to the upper posterior edge of the maxillaries and parallel with it a 

 very small bone called the supplemental maxillary. The relative size of the mouth is 

 conveniently indicated by describing how far back the maxillary extends; thus "max- 



