PAIRING, COURTSHIP, AND PARENTAL CARE 295 



which can be withdrawn into a shallow glandular pouch in 

 front of the fin; the head is surmounted by a curious club-like 

 appendage, the frontal or cephalic clasper, armed with a group 

 of curved spines, and this can be lowered into a depression in 

 the skin when not required (Fig. 80). Distinct marks and 

 scratches that have been observed on the skin of female 

 Chimaeras at the base of the dorsal fin are believed to have 

 been caused by the frontal claspers of the males, who probably 



Fig. 107. 

 Thornback Ray {Raia clavata), X i. 

 Ventral views of maJe and female. 



make use of these organs to retain their hold when curling their 

 bodies round those of their mates during coition. 



Apart from the mixopterygia, sexual differences are rare in 

 Selachians, but in many of the Rays [Raia) the males are 

 provided with a patch of sharp spines on the upper surface of 

 each pectoral fin, and this is entirely wanting in the other sex. 

 These cannot play any part in grasping the female, and it 

 seems likely that they serve as oflfensive weapons, for it is 

 known that several males will pursue one female, fighting 

 among themselves and buffeting each other with their fins. 

 In some species the form and arrangement of the spines on the 

 body and tail differs in the two sexes, and in the Thornback 



