xm 



PHYLUM CHORDATA 



225 



Oss 



JL-.J2J' 



pterygium (MS), and an ossified metapterygium (MT) ; to these, 



two rows of elongated radials (Ra, ^ 



Ra l ) are articulated fanwise, and 



these in their turn give attachment 



to the fin-rays (FS). In all the re- 

 maining orders the basalia (pro-, 



meso-, and meta-pterygium) are ab- 

 sent, and the endoskeleton of the fin 



consists only of a single or double row 



of radials (Fig. 874). 



In Polypterus there is a vestigial 



pelvic girdle (Fig. 900, BP) in the 



form of a small rhomboidal cartilage 



to which the anterior ends of the 



basalia (Bas 1 ) are attached : thus in 



the structure of the posterior ex- 

 tremities also, the Crossopterygii are 



the most primitive of the Teleostomi. 



In all the remaining orders the pelvic 



girdle appears to be atrophied. The 



pelvic fin is supported by a single 



bone of variable form (Fig. 875, 



B. PTG) and apparently arising from 



the fusion of proximal pterygio- 



phores. Between its posterior end 



and the dermal rays irregular 



nodules, representing radials, may be interposed. 



The distinction between hard or unjointed fin-rays, or spines, and 



soft or jointed fin-rays has already 

 been referred to. The first ray of 

 the dorsal and pectoral fins some- 

 times, e.g. in Siluroids (Fig. 887), has 

 the form of a very strong spine 

 articulated by a bolt-and-shackle 

 joint, i.e. by the interlocking of 

 two rings. In some cases the 

 first dorsal spine springs from the 

 skull. 



The texture of the bones is sub- 

 ject to w r ide variation : in some 

 Acanthopteri they are very thick 

 and strong, in some places almost 

 like ivory ; while in the Lump- 

 fish (Cyclopterus), the huge Sunfish 

 (Orthagoriscus), and in many deep- 

 sea forms, such as the Eibbon- 



fishes (Regalecus and Trachypterus), the amount of mineral matter 



FIG. 899. Pectoral fin of Polypterus. 

 FS. dermal rays ; MS. mesoptery- 

 gium ; MT. metapterygium ; NL, 

 nerve-foramina ; Oss. ossification in 

 mesopterygium ; Pr. propterygium ; 

 .Ra. first radials ; Ka 1 . second radials. 

 At * the bony marginal rays meet 

 and shut otf the middle region from 

 the shoulder-girdle. (From Wieders- 

 heim's Comparative Anato/nii.) 



BP 



FIG. 900. Pelvic fin of young Poly- 

 pterus. Ap. part of basale ; Baa 1 . 

 basale ; B/'. pelvic cartilages (fused 

 in adult) ; Rad. radials. (From 

 Wiedersheim.) 



