150 SELACHII 



keel is developed on each side of the tail. A pit is present at the base 

 of the caudal fin. The large pointed teeth are filled with osteodentine. 



In Cetorkinus (which is sometimes placed in a separate family) the 

 teeth are small and very numerous, and the branchial arches are provided 

 on both sides with a comb-like series of gill-rakers (Fig. 110). These 

 are enormously elongated denticles, uncalcified, and of horny consistency 

 (Turner [470]). Although strikingly like the gill-rakers of certain 



Fin. 111. 

 Lamna cornitbico., Gni. (From Jordan and Evermann.) 



Teleostei, yet they are of quite different origin. The deep-sea genus 

 Mitsukurina has a protractile mouth and spatulate snout. 



SUB-FAMILY 1. LAMXINAE. Orthacodus, A. S. W. ; Jurassic, Europe. 

 Scapanorhynchus, A. S. W., Cretaceous, Europe and Asia, is perhaps the 

 same as the living Mitsulmrina, Jordan, Japan. Odontaspis, Ag., Lamna, 

 Cuv. (Fig. Ill), and Oxyrhina, Ag., in tropical and temperate seas, 



FIG. 112. 

 Alopecias (Alojn!<) UMAX'S, Gm. (After Day, Fishes of India,) 



extend down to Cretaceous ; Alopecias, M. and H. (Fig. 112), and 

 Carcharodon, M. and H., to Eocene. Cetorhimis, Blainv. (Selache, Cuv.) 

 (Fig. 109) ; Atlantic, and Pliocene, Belgium. 



SUB-FAMILY 2. RHINODONTINAE. Khmodon, one of the largest sharks, 

 reaching the length of some 70 feet, has gill-rakers like Cetorhinus. The 

 nostrils are near the margin of the mouth, which is almost terminal. 

 The 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). 



