148 



SELACHII 



"<* 



\ 



FIG. 108. 



Spiral row of teeth of Helicoprion bessonovi, Karp. ; Permo-Carboniferous, Uussiu. A, new 

 teetli being formed ; B, teeth in use ; C, old teeth passed out of use. (After Karpinsky, from 

 Brit. Mas. Guide.) 



DIVISION B. 



Only the hyostylic type of jaw attachment is known to occur 

 in these fish. The notochord is always constricted. 



A rostrum is present, generally strengthened by cartilage ; it 

 appears, however, to have been lost in the Squatinidae and 

 Centrobatidae. 



SUBDIVISION 1. 



Sub-Order 1. SCYLLIOIDEI. 



An anal fin is present and the dorsal fins are spineless. The 

 centra are generally asterospondylous ; concentric calcifications may 

 appear (Cetorhimis), as well as the more usual radial calcifica- 

 tions, but there are generally four 'radiating wedge-shaped 

 masses of uncalcified cartilage converging towards the centre from 

 the bases of the arches (Fig. 94). The rostrum is supported by 

 three cartilages, which generally meet at a point in front (Fig. 59, 

 C). The palato-basal articulation is loose, and the process reduced. 

 The teeth remain fairly simple, the median cusp enlarging to a 

 sharp piercing cusp (Sphenodus), or a flattened conical cutting blade 

 (Carcharodon). The spiracle is small, or may be closed up. 



