CESTRACION. 213 



tooth is shown in PL XLIV, fig. 18, while an inner view of a similar tooth (fig. 19) 

 is added to display the groove for the nutritive foramen in the root. In a smaller 

 tooth (fig. 20) the denticles are especially long and slender, while the root is com- 

 paratively small. In a posterior tooth (fig. 21), which is wide in proportion to its 

 height, the pointed lateral denticles are broader. 



Horizons and Localities. — Zone of Belemnitella mucronata : Norwich. Zone of 

 Marsupltes : Thanet, Kent. Zone of Mlcrastev coranguinnni : Gravesend, Green- 

 liithe, and Thanet, Kent; South Croydon, Surrey; Grays, Essex. Tiironian 

 zones : near Lewes, Sussex ; Whyteleafe, Surrey. Zone of Ilolaster suhghbosus : 

 Burham and Hailing, Kent ; Arundel, Sussex. Zone of ScJtloenhachia varians : 

 Dover, Kent. Undetermined zones: Charing, Kent; Guildford, Surrey ; Heytes- 

 bury, Wiltshire. Also Upper Greensand, Cambridge Greensand, and Gault. 



Family Cestraciontid^. 



The Cretaceous Cestraciont sharks are known only by fragments. Cestmclon 

 itself, which ranges from the Upper Jurassic to the existing fauna, appears to be 

 represented; while Sijnecliodus is the sole survivor of the Hybodonts in the Chalk. 



Genus CESTBACION, Cuvier. 



Cestracion, G. Cuvier, Kcgiie Auimal, vol. ii, 1817, p. 129. 



Drepanephorus, P. M. G. Egertou, Figs, aud Descript Brit Organic Kemaius (Mem. Geol. Surv., 

 1872), dec. xiii, uo. 9. 



(xenerlc Characters. — Anterior dorsal fin arising in front of the pelvic pair, 

 posterior dorsal in front of the anal fin ; each with a smooth, enamelled dorsal 

 fin-spine, which does not bear posterior denticles. Mouth terminal, or nearly so ; 

 pterygo-quadrate cartilage in the adult directly connected Avitli the cranium by an 

 antorbital articulation. Anterior teeth small, numerous, cuspidate, generally with 

 one pair of lateral denticles ; lateral and postero-lateral teeth without cusps, but 

 relatively broad and flattened, with a more or less distinct longitudinal keel and 

 reticulate ornamentation. Shagreen fine and close; no head-spines; no large 

 dermal hooks on the claspers of the male. Vertebral centra well developed, 

 asterospondylic. 



Tijpe Species. — An existing shark, Cestracion ijhUlppi. 



Remarks. — The mandible of the type species, with its characteristic dentition, 

 is shown in Text-fig. 68. Here the two rami meet in a long symphysis, but in 

 some species the mandibular symphysis is short (so-called Lhjropleurodas, T. N. 

 Gill, Proc. Acad. Nat. Sci. Philad., 1809, p. 482). In this jaw of an adult fish. 



