XIII 



PHYLUM CHORDATA 



231 



and suspensorium is nearly vertical or even inclined backwards; 

 in small-mouthed forms (Fig. 880) it is strongly inclined forwards, 

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

 branchial arches the pharyngo-branchials of each side are very 

 commonly fused, and constitute what are called the superior 

 pharynycal bones : the re- 

 duced fifth branchial bars, 

 or inferior pharyngeal bones, 

 bite against them. The 

 Pharyngognathi are dis- 

 tinguished by having the 

 inferior pharyngeal bones 

 united into a single bony 

 mass of characteristic form 

 (Fig. 879, 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 Chon- 

 drostei approach the Elas- 

 mobranchs. 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 reduced in size and is 

 usually ossified by two 

 bones, a dorsal scapula and 

 a ventral coracoid: some- 

 times, as in the Trout, there 

 may he an additional ossi- 

 fication, the mesocoracoid. 

 Additional investing 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. 863). 



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

 approach most nearly to Elasmobranchs. In Polypterus (Fig. 

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



p 2 



Fig. 887. Skull of Polypterus, from above F, 

 frontal-; M. maxilla ; NA. nasal ; Na. nostril ; Op. 

 opercular ; Orb. orbit ; P. parietal. The remaining 

 letters points to less important investing bones. 

 The arrow is passed into the spiracle. (From 

 Wiedersheim's Comparative Anatomy.) 



