Fishes . ratenot articulated to the cranium. The notochord is mainlypersistent, in part replaced by feeble cyclospondylic vertebralcentra. Each gill-opening is bordered by a broad frill of is but one dorsal fin. The teeth closely resemble thoseof Dittodits or Didytnodus and other extinct Ichthyotomi. Theteeth have broad, backwardly extended bases overlapping,the crown consisting of three slender curved cusps, separatedby rudimentary denticles. Teeth of a fossil species, Chlamy-doselachus lawleyi, are recorded by J. \V. Davis from the Plioceneof Tuscany. Order Asterospondyli.—The ord
Fishes . ratenot articulated to the cranium. The notochord is mainlypersistent, in part replaced by feeble cyclospondylic vertebralcentra. Each gill-opening is bordered by a broad frill of is but one dorsal fin. The teeth closely resemble thoseof Dittodits or Didytnodus and other extinct Ichthyotomi. Theteeth have broad, backwardly extended bases overlapping,the crown consisting of three slender curved cusps, separatedby rudimentary denticles. Teeth of a fossil species, Chlamy-doselachus lawleyi, are recorded by J. \V. Davis from the Plioceneof Tuscany. Order Asterospondyli.—The order of Asterospondyli comprisesthe typical sharks, those in which the individual vertebrae arewell developed, the calcareous lamellas arranged so as to radiate,star-fashion, from the central axis. All these sharks possesstwo dorsal fins and one anal fin, the pectoral fin is normally The True Sharks 187 developed, with the three basal cartilages; there are five gill-openings, and the tail is Fig. 129.—Bullhead-shark, Heterodontus francisci (Girard). San Pedro, Cal. Suborder Cestraciontes.—The most ancient types may be setoff as a distinct suborder under the name of Cestraciontes orProsarthri.
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