SORTATION. 27 



fins. The brown shark (Notidanus griseus) has but one, and is also 

 recognisable by having six gill-slits, whereas all the other sharks 

 have five. That reduces our eleven to ten, which we can arrange 

 in two batches according to the position of their first dorsals. In 

 the first batch we will place those in which the first dorsal comes 

 over the space between the ventrals and the anal, that is, in the 

 hinder third of the back; in the other batch, we will have those 

 in which it comes over the interval between the pectorals and 

 ventrals, or almost in the middle of the dorsal curve. Those with 

 the dorsals well aft are two in number Pristiimis, in which the 

 anal is long enough to nearly reach the caudal ; and Scyllium, in 

 which it is not noticeably large, and never gets nearer the caudal 

 than its own length. Of Pristiurus there is a solitary species, 

 P. melanostomus,the black-mouthed dog-fish, recognisable at a glance 

 by the three rows of squarish black blotches on its sides that 

 suggest an ocelot; and of Scyllium, in which the markings are a 

 multitude of spots and specks, there are but two species S. 

 canicula, the rough-hound, and 5. catulus, the smooth-hound, in the 

 first of which the anal ends below the interval between the dorsals, 

 while in the second it reaches to beneath the middle of the second 

 dorsal and no further. 



Of the sharks in which the dorsals come so far forward for 

 the first to begin over the interval between the paired fins, one, 

 Zygana, stands out boldly, not only from the sharks but from all 

 other fish, by its hammer-shaped head, which extends right and 

 left at right angles to the neck, and has the eyes in the middle of 

 the square ends. These eyes have a lid or nictitating membrane, 

 and this membrane will come in useful in sorting out the six 

 remaining genera, in three only of which it is present. These are 

 Carcharias, Mustelus, and Gakus, and they all have long, conical 

 heads. The blue shark, Carcharias glaucus, has a pit at the base 

 of the tail, and is without the spiracles that mark the opening of 

 the respiratory passage leading into the pharynx. Mustelus and 

 Galeus have no pit at the base of the caudal fin, and in the former 

 the second dorsal is almost as large as the first, whereas in the 

 other it is very small. Of each there is but one species, M. vulgaris 

 being the smooth-hound, and G. vulgaris the tope which can 

 be distinguished from it by the squarish gap in the caudal's lower 

 lobe. 



Of the three genera in which the eye has no nictitating mem- 

 brane, one is as readily recognisable as the hammer-head by reason 

 of its enormously-developed tail, the upper lobe of which is as long 

 as the body. This is the thrasher (Alopecias), also known as the 

 fox-shark. The long tail has no keel, like that of the two remaining 

 genera Lamna, in which the teeth are large and lanceolate, the 

 British representative being the porbeagle, L.cornubica; and Selache, 

 represented by the huge basking shark, S. maxima, in which the 

 teeth are small and conical, as might be expected from the innocent 

 nature of its food. With the vegetarian Selache we conclude our 

 second category of sharks, which we tabulate in this way : 



Anal present 



One dorsal Notidanus. 



