462 FISHES CHAP. 



which they live, and thus concealed, small Fishes, Crustaceans, 

 and other organisms are lured unsuspectingly within the reach of 

 the comparatively inactive and sluggish Kay. From the ventral 

 position of the mouth the Kay cannot at once seize its prey, but 

 the Fish darts over its victim and covers it with its body, and 

 then readily devours it. The sexes are usually distinguished by 

 secondary sexual characters, which take the form of differences in 

 size and coloration, in the dentition, and also in the presence and 

 position of patches or rows of specially modified dermal spines on 

 the dorsal surface (Fig. 264). Some of the larger species reach 

 a great size, the disc measuring 7 to 8 feet in width. A few 

 species range into deep water. 1$. mamillidens, a uniformly jet- 

 black species, has been obtained from a depth of 597 fathoms in 

 the Bay of Bengal, 1 and H. abyssicola from 1588 fathoms off 

 Queen Charlotte Islands, British Columbia. 2 The following are 

 British species : the Thornback (R. clavata) ; the Spotted Kay 

 (H. maculata) ; the Painted Kay (R. microcellata) ; the Starry 

 Kay (R. radiata) ; the Cuckoo or Sandy Kay (II. circularis) ; 

 the Skate (R. batis) ; the Flapper Skate (E. macrorhynchus) ; the 

 White Skate (JR. alba) ; the Long-nosed Skate (H. oxyrhynclms) ; 

 and the Shagreen Kay ( R. fiillonica'). 3 Most of the species are 

 of some economic value as food Fishes. Psammobatis, with a 

 circular disc, frequents the southern coasts of South America, 

 and Platyrliina the coasts of India, China, and Japan. 



The family ranges from the Upper Cretaceous, in which, as 

 well as in different Tertiary deposits, it is represented by species 

 of Raia. An extinct genus, Cydobatis, with a circular or oval 

 disc, occurs in the Upper Cretaceous of Mount Lebanon. 



Fam. 4. Tamiobatidae. The systematic position of the only 

 representative of this family, Tamiobatis vetustus* from the 

 Devonian or Lower Carboniferous of Kentucky, is very uncertain, 

 but in some respects this unique type seems to be intermediate 

 between the modern Sharks and the Kays. 



Fam. 5. Torpedinidae (Electric Kays). A disc is formed as in 

 the Kaiidae, but it is sub-circular in shape rather than rhombic, 

 and in the nature of its endoskeletal supports it is in some 

 respects unique. Its semicircular anterior margin is supported 



1 Alcock, Ann. Mag. Nat. Hist. (6), iv. 1889, p. 380. 



2 Jordan and Evermaun, op. cit. p. 76. 



3 Day. op. cit. p. 336. 4 Zittel, op. cit. p. 41. 



