xni 



PHYLUM CHORDATA 



281 



and suspensorium is nearly vertical or even inclined backwards; 



in siiiall-ni()utlu'(l forms (Fit;". (SSO) it is strongly inclined forwards, 



and the length of the jaws is propoi-tionately reduced. In the 



branchial arches the pharyngo-branchinls of each side are very 



commonly fused, and constitute what are called the superior 



pharyngeal hones : the re- 

 „i,;„i u — „ 



J'm.f 



duced fifth branchial bars, 

 or inferior pilar jingeal honoi, 

 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 

 cartilases, 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 sccqnda and 

 a ventral coracoid: some- 

 times, as in the Trout, there 

 may be an additional ossi- 

 fication, the mesocoracoid. 

 Additional investing bones 



— siipra-claricle, 2^ost-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. 

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



p 2 



Fig. SS7.— Skull of Polypterus, fi-om above F, 

 frontal ; M. maxilla ; a\'A. nasal ; Nii.. nostril ; Op. 

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

 letters i^oints to less important investing bones. 

 The arrow is passed into the spiracle. (From 

 Wiedersheim's Comparative Anatomy.) 



