THE SPINY DOGFISHES AND THEIR ALLIES 2909 



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

 Whereas in the gray comb-toothed shark (Notidanus griseus) of the Atlantic and 

 Mediterranean, 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 are 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. 



From the typical genus of the family the Japanese frill-gilled 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 teeth are also of a somewhat simpler structure, being similar in both jaws, and 



each consisting of three slender, curved, and subconical 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 DOGFISHES AND THEIR ALLIES Family 



Although the members of the present family approximate in their external con- 

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

 vertebras they agree with the latter. Accordingly, both the spiny dogfishes, rays, 

 sawfishes, and their kindred are regarded as forming a suborder (Tectospondyli) 

 distinguished from the one including the preceding families by the following char- 

 acteristics: In the bodies of the vertebras, when fully developed, the concentric 

 calcified plates are more numerous than those radiating from the centre; and the 

 anal fin is invariably wanting. In the more specialized forms the body is greatly 

 depressed, and the pectoral fins attain an enormous development; while the spiracles 

 are of large size, and always retained. The present family includes the most gen- 

 eralized members of the group, in which the body is cylindrical or triangular, and 

 but very slightly depressed; the mouth being gently arched, and the muzzle blunt. 

 The pectoral fins have no forward prolongation, and are not notched at their point 

 of origin; and the small and lateral gill clefts may be either in the line of the pec- 

 torals, or half below. The large spiracles are placed behind the eyes; there is no 

 nictitating membrane to the eye; and the two dorsal fins may or may not be pro- 

 vided with spines. 



The common spiny, or picked, dogfish (Acanthias vulgaris], 

 shown in the upper figure of the illustration on p. 2900, is the 

 most familiar representative of a very small genus characterized by the presence 

 of spines to the dorsal fins, and by the peculiar form of the teeth, which are 

 similar in the two jaws, and small, triangular, and compressed, with the points 

 much turned aside, and the cutting edge formed by the inner margin. The com- 

 mon species measures from three to four feet in length, and is slaty blue abov.e, and 



