232 



ZOOLOGY 



SECT. 



propterygiuni 



^a 



of Polypterus 



Ma. mesoptcry 



Fig. SSS.— Pectoral fin 

 F&. dermal rays ; 

 glum ; MT. mctapterj-giuDi ; Ai, 

 iiorve-foraiiiina ; 0.«. ossification in 

 mesopteryginm ; Pr. propterygium ; 

 Ra. first r idials ; Ha!, second radials. 

 At * the bony marginal rays meet 

 and shnt off the middle region from 

 the shoulder-girdle. (From Wicder- 

 sheim's Comparative Anatomt/.) 



arising 



broad cartilaginous, partly ossified, meso- 

 pterygium (MS), and an ossified 

 metapterygium (MT); to these, two 

 rows of elongated radials (lia, Ra^) 

 are articulated fjxnwise, and these 

 in their turn give attachment to 

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

 maining oiders the basalia (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 

 pride girdle (Fig. 889, 5P) in the 

 form of a small rhomboidal cartilage 

 to which the anterior ends of the 

 basalia (Bas^) 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 ^^c/vic fin is sup- 

 ported by a single bone of variable 

 form (Fig. 864, bstg) and apparently 

 representing a hasale, i.e. a structure 

 fusion of proximal pterygiophores. Between 

 and the dermal rays irregular nodules, repre- 



-FJ" 



r&OA 



from the 

 its posterior end 

 senting radials, may be interposed. 



The distinction between hard or 

 unjointed fia-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.ff. in Siluroids (Fig. 876), 

 has the form of a very strong spine 

 articulated by a bo!t-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 

 (Cijclojitcru-s), the huge Sunfish 

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



Fig. SS9.— Pelvic fin of yomig Poly- 

 pterus. .-(/). part of basale ; Tiit.v'. 

 liasalc ; }il'. pelvic cartilages (fused in 

 adult) ; Had. radials. (From 

 dersheim.) 



Wio- 



