THE SHARKS AND^RAYS 99 



touched, leaving dull white blotches wherever the colour-pig- 

 ment is destroyed. This might possibly be turned to useful 

 account in removing ink or other stains from deal tables ; and 

 as the skin is actually used in polishing, this property may 

 perhaps assist its roughness in attaining the desired object. 

 Whether it is retained for any considerable period after death, 

 and whether the dried skin would, on being moistened, recover 

 its virtues, is an experiment that would perhaps be worth 

 trying. 



The Black-mouthed dog-fish {Tristiiirus melamstoma), 

 characteristically a southern form, has two peculiarities that 

 serve to distinguish it from other of our sharks. In the first 

 place, as its name denotes, the interior of its mouth is dark 

 (though not always black), and in the second the upper edge of 

 the tail-fin is serrated with rows of small spines. There is also 

 a fold of skin on the snout, but something similar is also 

 apparent in the nurse. In colour this shark is dull yellow 

 (the plate given by Couch is far brighter in hue than any 

 examples caught by the writer on the west coast of Morocco, 

 at Casablanca), and there are black spots along the sides. It has 

 generally been regarded as a straggler only in British seas, but 

 several were both hooked and trawled during the Irish survey 

 of 1890-91, in water varying from 150 to 250 fathoms, a fact 

 which led Holt and Calderwood to regard it as possibly a 

 deep-water shark with us, though a littoral species in the 

 warmer Mediterranean. A female was trawled off Aberdeen, 

 November, 1898, and the egg-case was found to have tendrils 

 at the lower end. On parts of the Morocco coast it is 

 common enough, examples of 2 or 3 ft. being hooked in the 

 spring and summer in less than 20 fathoms. 



Notidanidae 



In all the sharks previously noted, and in all those, and the 

 rays, which follow, there are but five gill-openings. In the 

 present family there may be either six or seven. The only 



