FRILL-GILLED SHARK. 533 



teeth of the lower than in those of the upper jaw. With the occasional exception 

 of some portions of the tail, the notochord persists throughout life. At the present 

 day the range of the existing- members of the genus includes most temperate and 

 tropical seas, some of the species reaching as much as 15 feet in length. Whereas 

 in the grey comb-toothed shark {Notidanus griseus), of the Atlantic and Mediter- 

 ranean, the number of gill-clefts is six, in each of the other three species it is seven. 

 Fossil species occur from the Pliocene to the middle Jurassic: many of these, like 

 the one of which two teeth arc shown in the illustration, being of much larger 

 dimensions than any of the existing forms. As to the habits of these sharks, there 

 appears to be practically no information. 



Friii-Giiied From the typical genus of the family the Japanese frill-gilled 



Shark. shark differs by the greatly elongated and slender form of the body: 



and by each of the six gill-clefts being protected by a frill-like flap of skin. The 



FRILL-GILLED SHARK. 



teeth are also of a somewhat simpler structure, being similar in both jaws, and each 

 consisting of three slender, curved, and subcorneal cusps, separated by a pair of 

 rudimentary ones : while there is an unpaired median series at the extremity of 

 the lower jaw only, instead of in both the upper and the lower. Although mainly 

 persistent, the notochord is in part replaced by ill-developed vertebrae of the type 

 characteristic of the suborder. Fossil teeth from the European Miocene have been 

 assigned to this genus. 



The Spiny Dog-Fishes and thetr Allies, — Family Spinacid^E. 



Although the members of the present family approximate in their external 

 conformation more to the typical sharks than to the rays, yet in the structure of 

 their vertebrae they agree with the latter. Accordingly, both the spiny dog-fishes, 

 rays, saw-fishes, and their kindred are regarded as forming a suborder (Tecto- 

 spondyli) distinguished from the one including the preceding families by the 

 following characters. In the bodies of the vertebrae, when fully developed, the 

 concentric calcified plates are more numerous than those radiating from the centre : 



