The True Sharks 



5^5 



younger than the Cestraciontes and Httle older than the Lam- 

 noids, or the Squaloid groups. Heptranchias niicrodon is com- 

 mon in EngHsh Cretaceous rocks, and Heptranchias primigenius 

 and other species are found in the Eocene. 



Family Chlamydbselachidse. — Very great interest is attached to 

 the recent discovery by Samuel Garman of the frilled shark, 

 Chlamydoselachus anguineiis, the sole living representative of 

 the Chlamydoselachidce. 



Fig. 314. — -Frill-shark, Chlamydoselachus anguineus Garman. From Misaki, 

 Japan. (After Gunther.) 



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, Chlamy- 

 doselachus lawleyi, axe 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 vertebrae are 

 well developed, the calcareous lamellae 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 



