A treatise on zoology . 2. Rhinodoxtinae. hhinodon, one of the largest sharks,reaching the length of some 70 feet, has gill-rakers like Cetorhinus. Thenostrils are near the margin of the mouth, which is almost minute conical teeth are very numerous. Rhinodon, Smith ; southern and tropical seas. Family Carchariidae. Usually pointed hollow teeth. No oro-nasal grooves. The spiracle may be absent {Carcharias, Sphyrninae). SQUALIFORMES 151 A third eyelid, or nictitating membrane, is present (Fig. 113); itappears to be a specialised anterior region of the lower lid. Sub-Family 1. Carcha
A treatise on zoology . 2. Rhinodoxtinae. hhinodon, one of the largest sharks,reaching the length of some 70 feet, has gill-rakers like Cetorhinus. Thenostrils are near the margin of the mouth, which is almost minute conical teeth are very numerous. Rhinodon, Smith ; southern and tropical seas. Family Carchariidae. Usually pointed hollow teeth. No oro-nasal grooves. The spiracle may be absent {Carcharias, Sphyrninae). SQUALIFORMES 151 A third eyelid, or nictitating membrane, is present (Fig. 113); itappears to be a specialised anterior region of the lower lid. Sub-Family 1. Carcharinae. With an elongated rostrum. Mxidelushas acquired a blunt crushing dentition. Carcharias, Cnv., and Galeocerdo, M. and H., of world-wide distribution,extend down to Eocene in the Old and the New World. Galeus, Cuv.,and Mustclus, Cuv.; distribution ^^ general, and to Eocene, Eurojie. \ Hemiprisfis, Ag. ; to Eocene,Europe, Asia, and N. America. Sub-Family 2. Sphyrninae. M?- ^ W i^ ^A Specialised forms in which the ^. head is produced at the sidesinto flat processes carrying theeyes and nostrils outwards. Thecartilaginous skull is correspond- t/ ^ ^\, .^f. iiigly modified, large extensions . /^ of the pre-and i^ostorbital regions Fio. 113. supporting the eyes. Left eye of Mustelns laevis, Risso. H, lower In Sphyrmt the process is *y«^ ^P sPracle ; , third eyelid or nictitating., T , r, , , membrane; , upper eyelid, only moderately developed, but in the Hammer-headed Zygaena 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 (Acanfhias, Fig. 50).Sometimes they are tectospondylous, with numerous complete con-centric cylinders (Bhina, Fig. 52); or again they may have radiatingcalcifications (Baja). Only the hyostylic type of skull i
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