THE SHARKS AND RAYS 97 



as migratory on our coasts there is some uncertainty, but the 

 largest examples are said to occur only in the warmer 

 months. 



The Smooth-hound {Mustelus vulgaris) is in outline 

 something like the last, only there is no lower lobe to its tail 

 and the upper part of its grey body is marked with indistinct 

 light spots. Otherwise, it has the same nictitant membrane 

 and spiracles, the same notch on the upper part of the tail-fin, 

 and the same rough skin ; and it also grows to about 6 ft. in 

 length. There is a closely aUied shark (iVf. lavis) which is 

 of great interest on account of a placental connection between 

 the unborn young and the parent ; but whether this species is 

 actually found on our coasts seems doubtful, though it would 

 in all probability come within the British area. The teeth 

 of the smooth-hound are flatter and blunter than those of the 

 rest of the British members of the family, so that it may feed 

 on shell fish. It produces living young, though there is no 

 placental connection in the species commonly met with on our 

 shores. It must not be forgotten that a placental connection 

 is also noted in at any rate some species of Zygana and 

 Carcharias, but not in such viviparous rays as Trygon and 

 Myliobatis.* 



Scylliidae 



The sharks belonging to this family are chiefly interesting 

 because they deposit eggs like most of the rays, which eggs 

 take about eight or nine months to hatch out in captivity. 

 How far this may be accepted as an index of the time taken 

 under natural conditions is a matter of opinion, but it is 

 always important to make some reservation. All three 

 British representatives of the family are only small sharks, 

 and are more or less spotted. The eye has no nictitant 

 membrane, the teeth are small, and there are breathing 



* See Alcock, "On the Gestation of Elasmobranch Fishes" (Journ. 

 Asiat. Soc. of Bengal, 1890, Vol. LIX., Part II., pp. 51-56). 



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