SQUAL1FORMES 151 



A third eyelid, or nictitating membrane, is present (Fig. 113); it 

 appears to be a specialised anterior region of the lower lid. 



SUB-FAMILY 1. CARCHARINAE. With an elongated rostrum. Mustelus 

 has acquired a blunt crushing dentition. 



Carcharias, Cuv., and Galeocerdo, M. and H., of world-wide distribution, 

 extend down to Eocene in the Old and the New World. Galeus, Cuv., 

 and Mustelus, Cuv. ; distribution ^ 



general, and to Eocene, Europe. 

 Hemipristis, Ag. ; to Eocene, 

 Europe, Asia, and N. America. 



SUB-FAMILY 2. SPHYRNINAE. 

 Specialised forms in which the 



head is produced at the sides , ^^5*^6^5^*^^:^-^ 

 into flat processes carrying the "- * 



eyes and nostrils outwards. The 

 cartilaginous skull is correspond- tl \ */.?. 



ingly modified, large extensions H 



of the pre-and postorbital regions FIG. us. 



supporting the eyes. Left eye of Mustd-ius laevis, Risso. II, lower 



In Sphyrna the process is e y eli( ' ; S P> s Pi, racle ; **. third eyelid or nictitating 



r y r membrane ; u.l, upper eyelid, 



only moderately developed, but 



in the " Hammer-headed" Zyyaena it is extraordinarily large (Fig. 114). 



Sphyrna, Raf., and Zygaena, Cuv. (sometimes united in one genus) ; 

 tropical and subtropical seas, Miocene, Europe, and N. America. 



SUBDIVISION 2. 



Without anal fin. The vertebral centra are often cyclospondylous, 

 with a simple constricted calcified cylinder (Acanthias, Fig. 50). 

 Sometimes they are tectospondylous, with numerous complete con- 

 centric cylinders (Rhina, Fig. 52) : or again they may have radiating 

 calcifications (Raja). Only the hyostylic type of skull is known. 

 The spiracle is well developed. The nostrils are usually separated 

 from the mouth. 



Sub-Order 1. SQUALIFORMES. 



These retain the shark-like body, with its large swimming tail. 

 The branchial slits are never ventral. 



Family SPINACIDAE. The gill-slits extend not more than half-way 

 below the level of the pectoral girdle. Vertebrae cyclospondylous 

 (Acanthias, Fig. 50 ; Spinax, etc.) ; or uncalcified, and with very large re- 

 mains of the notochord (Laemaryus, Echinorhinus [487]). The teeth are 

 generally of moderate size ; frequently they are more modified in the 

 lower than in the upper jaw (Spinax, Laemargus) ; and their points 

 are often turned aside so that the inner margin forms the cutting edge 

 (Fig. 39). 



A large hollow spine, fixed on the anterior cartilaginous radial, is 



