MODIFICATIONS OF THE JAWS OF FISHES. 53 



maxillary is drawn forwards out of its epicranial groove. 

 This action is aided by the hypotympanic, which is of 

 great length, and has a movable articulation at both ends ; 

 the lower end joining the mandible is pulled forward, 

 simultaneously with the protrusion of the premaxillary, 

 and co-operates therewith in the sudden projection of the 

 mouth, by which the sly-bream seizes, or shoots with a 

 suddenly -propelled drop of water, the small agile aquatic 

 insects that constitute its prey. 



An opposite extreme of modification of the maxillary 

 and premai:illary bones, where unusual fixity and strength 

 are needed, is that presented by the " sword-fishes," in 

 which the premaxillaries constitute, by an unusual pro- 

 longation and density of tissue, the sword-shaped weapon 

 characteristic of the genera Xiphias and Isiiophorus. 



In Cut 9 the divisions 28 a, c, and d^ of the tympanic 

 pedicle, and the two chief divisions, 29 and 33, of the 

 mandible, are shown, together with the four bones of the 

 opercular appendage ; the preopercular, 34, being serrated 

 and spined, as in most perches. 



Of the hyoidean arch may be seen the glossohyal, 42, 

 the ceratohyal, 40, with its branchiostegal rays, 44, and 

 the urohyal, 43. Of the scapular arch, the scapula, 51, 

 and the coracoid, 52, this supports not only the bones of 

 the " pectoral fin," P, viz : ulna, radius, with the small 

 carpal bones intervening between them and the metacar- 

 pophalanges, 57, but also the lower elements of the pelvic 

 arch, 63, and their diverging appendage, 69, called the 

 " ventral fin," Y. 



In the segments of the trunk the ha^mapophyses, save 

 in the first vertebra, 58, and the pelvic vertebra, 63, are 

 not ossified; but they are represented by aponeurotic 

 fascia continued downwards from the ossified elements of 



