308 



CHONDROSTEI 



basidorsals (neural arches) and small interdorsals (Figs. 280, 284). 

 The former have ossified dorsal extremities embracing the powerful 



B. 



,nt. 



nt.-- 



he. 



FIG. 284. 



mg. 



Vertebral column of Acipenser sturio, L., from the trunk region. A, left-side view of a piece 

 partly cut through longitudinally ; B, the same cut transversely, bv, basiventral ; h.c, haemal 

 canal; id, interdorsal ; it; interventral ; li, longitudinal ligament; na, basidorsal ; n.c, neural 

 canal ; n.sp, neural spine ; nt, notochord ; r, pleural rib ; sh, notochordal fibrous sheath bounded 

 outside by the elastica externa. 



longitudinal ligament, and to which articulate the long neural 

 spines. Ventrally runs a cartilaginous haemal tube formed by 



a corresponding series of basi- 

 ^ ventrals and interventrals, and 



surrounding the aorta. The 

 neural arches and spines of 

 the Palaeoniscids seem to 

 have been similar (Coccolepis). 

 The endoskeleton of the 

 median fins resembles rather 

 that of the Selachian than 

 that of the Holostei (Fig. 280), 

 In the dorsal and anal it is in 

 the form of segmented rods, 

 of which the two proximal 

 pieces are long and sometimes 

 ossified, while the distal seg- 

 ments are very much shorter 

 (Bridge [56]). There is little 

 concrescence, but probably 

 considerable concentration, as 

 the radials are more numerous 

 than the segments in which 

 they lie. Primitive character 



is also shown in the overlapping of the distal ends of the radials by 

 the base of the lepidotrichia, which are much more numerous. In 



Oblique ventral view of the head of Gonatodus 

 jmnctatus, Ag. ; Calciferous Sandstone, Wardie. 

 (After Traquair.) a.g, enlarged anterior lateral 

 gular; cl, clavicle ; ct, cleithrum ; l.g, lateral gtilar ; 

 m.g, median gular ; , nostril ; o, orbit. 



