296 



POLY PTERIN J 



and outer cartilage nodules (Fig. 263). Budgett [67] has shown 

 that the fin -skeleton is really of the pleurorachic rhipidostichous 

 type (p. 106), similar to that of the Selachii and lower Actino- 

 pterygii (Amia, p. 302). The posterior basal represents the axis 

 or metapterygium ; the anterior basal, the median plate, and the 

 distal elements being derived from the preaxial radials partially 

 fused at their base (Fig. 268). Still more Actinopterygian is the 

 skeleton of the pelvic fin ; here only four bony radials are preserved, 

 separately articulating with the pelvic girdle. Two long bones 

 with some small anterior cartilages represent the girdle (Fig. 

 269). 



The axial endoskeleton is well ossified. The notochord is very 

 much constricted by the solid bony amphicoelous centra (Fig. 265a). 



The neural arches are continuous 

 with the neural spines above. 

 Throughout the abdominal region 

 each centrum bears a pair of true 

 dorsal ribs stretching outwards in 

 the transverse septum to the skin, 

 and a pair of ventral or pleural 

 ribs below (p. 68) ; the former 

 increase in length forwards, the 

 latter increase in length back- 

 wards, and pass gradually into 

 haemal arches. The extremity of 

 the vertebral column stops far 

 short of the edge of the caudal 

 fin ; it is almost, but not quite, 

 perfectly straight, the end of the 

 notochord being just a little turned 

 upwards both in the larva and in 

 the adult (Koelliker [2 71], Budgett 

 [67]), and enclosed with the nerve- 

 cord in a cartilage sheath, as in 

 Lepidosieus and Amia. Although 

 the tail is outwardly symmetrical, 

 there is reason to believe that it is not truly diphycercal, but has 

 been derived from a more heterocercal form. For, while the dorsal 

 spines are separate from the radials in the epichordal lobe, in the 

 hypochordal lobe the lepidotrichia rest on direct prolongations of 

 the haemal arches as in typical heterocercal fins (Fig. 61, p. 101). 

 Two points of interest are to be noticed in the anal fin : the proximal 

 segments of the radials still articulate for the most part with the 

 haemal spines, and some of them fuse together, as in the median 

 fins of some Osteolepidoti (Fig. 2.3 2) ; and the dermal rays are still 

 much more numerous than the radials (Fig. 265a). In the caudal 



Fig. 268. 



Reconstruction of the pectoral girdle and 

 in-skeleton of a larval Polypterus, enlarged. 

 (After Budgett.) ■•, coracoid region ; /, fora- 

 men; »i, tnesopterygial cartilage plate; mt, 

 lnetapterygium ; pr, propterygium ; r, radial ; 

 se, scapular region. 



