224 



ZOOLOGY 



SECT. 



in wide-mouthed Fishes (Fig. 888) the axis of the hyomandibular 

 and suspensorium is nearly vertical or even inclined backwards ; 

 in small-mouthed forms (Fig. 891) it is strongly inclined forwards, 

 and the length of the jaws is proportionately reduced. In the 

 branchial arches the pharyngobranchials of each side are very 

 commonly fused, and constitute what are called the superior 



pharyngeal bones : the re- 

 duced fifth branchial bars, 

 or inferior pharyngeal bones, 

 bite against them. The 

 Pharyngognathi are dis- 

 tinguished by having the in- 

 ferior pharyngeal bones united 

 into a single bony mass of 

 characteristic form (Fig. 890, 

 B). The gill- rakers are often 

 very highly developed, and 

 may form a mesh capable of 

 retaining even microscopic 

 organisms. 



In the shoulder-girdle, as in 

 the skull, the Chondrostei 

 approach the Elasmobranchs. 

 There is a primary shoulder- 

 girdle consisting of large 

 paired cartilages, not united 

 in the middle ventral line, 

 and unossified : each is 

 covered externally by a large 

 scute-like investing bone, the 

 clavicle. In the remaining 

 Ganoids and in Teleostei the 

 primary shoulder-girdle is re- 

 duced in size and is usually 

 ossified by two bones, a 

 dorsal scapula and a ventral 

 FIG. 898. skull of Poiypterus, from above. F, coracoid : sometimes, as in 



frontal; M. maxilla; N. nasal; Na. nostril; the Trout there TYiav hp an 



Op. opercular ; Orb. orbit ; P. parietal. The re- \ ,. V ,' l [1< y ' 



maining letters point to less important investing additional OSSlficatlOn, the 



bones. The arrow is passed into the spiracle 7 * i TJ_* i 



(From Wiedersheim's Comparative Anatomy.) ' meSOCOmCOld. Additional m- 



vesting bones supra-clavicle, 



post-clavicle, &c. are added, and one of them, the post-temporal, 

 serves to articulate the shoulder-girdle with the skull (Fig. 874). 



In the skeleton of the pectoral fin it is the Crossopterygii which 

 approach most nearly to Elasmobranchs. In Poiypterus (Fig. 

 899) the basal lobe of the fin is supported by a rod-like ossified 

 propterygium (Pr), a broad cartilaginous, partly ossified, meso- 



