APPENDICULAR SKELETON FISHES 



239 



fin, which forms a second symphysis by meeting its fellow. Occa- 

 sionally the anterior symphysis is lost, or the two symphyses may 

 lengthen so that the two halves meet in their whole length. 



In most Ganoids and Teleosts these structures do not form a true 

 pelvis, the basal parts of the fin being, as Chondrostei show, basip- 

 terygia, formed of the basal parts of several radialia, and the median 

 cartilage of Polypterus and some Sturionids may be the only true 

 pelvic structure. Only on such a basis can the development of the 

 girdle in Elasmobranchs and higher fishes be compared. The posi- 

 tion of the pelvic fins in 'Thoracici' and 'Jugulares' was referred 

 to above (p. 228), the pectoral and pelvic girdles being connected by 

 ligaments or occasionally (Astroscopus) may be firmly united. 



The paired fins of Teleostomes differ from those of Elasmobranchs 

 in several points. The number of radialia and their size is reduced, 

 and, except in a few Ganoids and Dipnoi, there is little in the pectoral 

 fin to compare with the three basal plates of sharks, most of the 

 radials articulating directly with the girdle. The 

 fins themselves are largely supported by flexible, 

 Httle ossified lepidotrichia hke those of the 

 median fins; they are usually divided trans- 

 versely into small blocks. Ceratotrichia may 

 also be present. 



The pectoral fins of Ganoids have both pro- 

 and metapterygia; those of Crossopterygians 

 (fig. 252) have a large mesopterygial carti- 

 lage, slightly ossified in the centre, between 

 them. In other genera the place of this is 

 taken by several radials which articulate 

 directly with the girdle. Acipenser has a large 

 dermal bone on the preaxial side of the fin, 

 with which the propterygium is closely con- 

 nected; this is followed by several radials, more 

 numerous .dis tally, as in many Elasmobranchs. 

 anterior margin of the fin has an ossified spine. 



The ventral fin of most Ganoids has a large basal element (called 

 a basipterygium, in Acipenser more Hke a propterygium) apparently 

 formed by the fusion of several basalia, and bearing a few radials, 

 sometimes ossified only in the middle. Calamoichthys lacks ventral 

 fins. 



Fig. 252.- — Pectoral fin 

 of Polypterus (Gegenbaur, 

 '64) ; bone white, cartilage 

 stippled. 7ns, mesoptery- 

 gium; mt, metapterygium; 

 p, propterygium; r, radi- 

 alia. 



Sometimes the 



