232 



ZOOLOGY 



SECT. 



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

 tp pterygium (MS), and an ossified 



metapterygium (MT) ; to these, two 

 rows of elongated radials (Ra, fia 1 ) 

 are articulated fan wise, and these 

 in their turn give attachment to 

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

 maining orders the basal ia (pro-, 

 meso-, and meta-pterygium) are 

 absent, and the endoskeleton of the 

 fin consists only of a single or double 

 row of radials (Fig. 863). 



In Polypterus there is a vestigial 

 pelvic girdle (Fig. 889, BP) in the 

 form of a small rhomboidal cartilage 

 to which the anterior ends of the 

 basalia (Has 1 ) are attached : thus in 

 the structure of the posterior ex- 

 tremities also, the Crossopterygii are 

 the most primitive of the Teleo- 

 stomi. In all the remaining orders 

 the pelvic girdle appears to be 

 atrophied. The pelvic fin is sup- 

 ported by a single bone of variable 

 form (Fig. 864, BSTG) and apparently 

 representing a lasale, i.e. a structure 

 arising from the fusion of proximal pterygiophores. Between 

 its posterior end and the dermal rays irregular nodules, repre- 

 senting 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. 876), 

 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 wide variation : in some 

 Acanthopteri they are very thick 

 and strong, in some places almost 

 like ivory ; while in the Lump-fish 

 (Cydopterus), the huge Sunfish 

 (Orthagoriscus), and in many deep-sea forms, such as the Eibbon- 



Fio. 888. Pectoral fin of Polypterus. 

 FS. dermal rays ; MS. mesoptery- 

 gium ; MT. metapterygium ; NL, 

 nerve-foramina ; Os*. ossification in 

 mesoptcrygiurn ; Pr. propterygium ; 

 Xa. first radials ; Ra'. second radials. 

 At * the bony marginal rays meet 

 and shut off the middle region from 

 the shoulder-girdle. (From Wicder- 

 sheim's Comparative Anatomy.) 



BP 



FlG 



fW 



9. Pelvic fin of young Poly- 

 pterus. Ap. part of basale ; Has*. 

 basale ; BP. pelvic cartilages (fused in 

 adult) ; Rad. radials. (From Wie- 

 dersheim.) 



