THE SHARKS AND RAYS 



i°5 



Raiidae 



Much of the confusion which has arisen between the 

 species of Raia is due to the great difficulty of diagnosing 

 from constant characters. Even in their colouring, which 

 the older writers describe with the utmost precision, in- 

 dividuals of a species differ widely, particularly if we 

 examine a series of different sexes and ages. Similarly, those 

 features that have long been regarded (and rightly, in a 

 measure) as secondary sexual characters, such as the form 

 and number of the teeth, abundance and distribution of 

 tubercles on the skin, or degree of curve in the fore part 

 of the disc, are subject to almost endless variation and quite 

 unreliable as indications of species unless accompanied by full 

 particulars of age. 



The exact relationship between the existing sharks and 

 rays is not, as already remarked, satisfactorily determined, 

 and we can only at present regard them as descendants from 

 a common stock. 



While it seems unnecessary to treat all the species of Raia 

 in detail, a few notes on each may be of use. 



