80 A NATURALIST ON DESEET ISLANDS. 



like selachians were descended from a common ancestor. 

 The "parting of the ways," between the two lines of 

 descendants, must have taken place at a very remote period 

 of time ; for well-developed representatives of both the rays 

 and the devil-fish famihes were living in the chalk period. 

 But these devil-fish are no more like a shark now than a 

 skate is. To discover " the secret of their birth," we must 



EABLY SHABK-LIKE EMBRYO OF THE INDIAN BAT-EAY {PteroplOtcea 



micrura), twice natural size, after alcock. 



have recourse, if we may be excused for an " Irishism," to 

 a study of their structure before they were born. In other 

 words, to embryology. Mr. Alcock in that most inter- 

 esting book of his, "A NaturaUst in Indian Seas," has 

 given us a gHmpse of their ancestry, as the res alt of his 

 researches in connection with the short-tail bat-ray 



