Fig. 314.—Frill-shark, Chlamydoselachus anguineus Garman. From Misaki, Japan. (After Günther.)
This shark was first found on the coast of Japan, where it is rather common in deep water. It has since been taken off Madeira and off the coast of Norway. It is a long, slender, eel-shaped shark with six gill-openings and the palato-quadrate not articulated to the cranium. The notochord is mainly persistent, in part replaced by feeble cyclospondylic vertebral centra. Each gill-opening is bordered by a broad frill of skin. There is but one dorsal fin. The teeth closely resemble those of Dittodus or Didymodus and other extinct Ichthyotomi. The teeth have broad, backwardly extended bases overlapping, the crown consisting of three slender curved cusps, separated by rudimentary denticles. Teeth of a fossil species, Chlamydoselachus lawleyi, are recorded by J. W. Davis from the Pliocene of Tuscany.
Order Asterospondyli.—The order of Asterospondyli comprises the typical sharks, those in which the individual vertebræ are well developed, the calcareous lamellæ arranged so as to radiate, star-fashion, from the central axis. All these sharks possess two dorsal fins and one anal fin, the pectoral fin is normally developed, with the three basal cartilages; there are five gill-openings, and the tail is heterocercal.
Fig. 315.—Bullhead-shark, Heterodontus francisci (Girard). San Pedro, Cal.
Suborder Cestraciontes.—The most ancient types may be set off as a distinct suborder under the name of Cestraciontes or Prosarthri.
Fig. 316.—Lower jaw of Heterodontus philippi. From Australia. Family Heterodontidæ. (After Zittel.)
These forms find their nearest allies in the Notidani, which they resemble to some extent in dentition and in having the palato-quadrate articulated to the skull although fastened farther forward than in the Notidani. Each of the two dorsal fins has a strong spine.
