NOTIDANID SHARKS. 27 



hyoid arch, which is much more slender than is usual in Sharks (see 



skull of Notidanus, 13, and compare with the hyostylic skull of 



Scyllium, 18). The mouth is inferior; the gill-clefts are six in 



number on each side in Hexanchus 



and seven in Heptanchus ; there are 



no flaps or frills over the gill-clefts. 



The teeth have a characteristic form 



(see 15, and fig. 13), and in the 



it io m^"T~~7^-7 lower iaw only one row of them is in 



-blG. 13. — Tooth of i\ otidanus J . 



gigas. use at a time. The principal teeth in 



their most perfect development pos- 

 sess a number of backwardly-sloping compressed cusps arising 

 from a long base; the anterior edge of the first cusp is finely 

 serrated. The teeth at the front of the upper jaw are smaller, 

 and have each a single awl-shaped cusp with one or more small 

 lateral cusps. The teeth of Notidanus are found in rocks as old as 

 the Jurassic ; examples are shown (16 a and b) of fossil teeth of 

 Tertiary age. The Notidanid Sharks attain to a length of fifteen 

 feet, and are distributed over the tropical and sub-tropical seas. 

 In Wall-case 1 is shown a small specimen of the Perlon or Seven- 

 gilled Shark, Notidanus (Heptanchus) indicus, 14, and suspended 

 from the rail in front of the case is a Grey Shark, or Six-gilled 

 Shark, Notidanus (Hexanchus) griseus, 1141, caught off the 

 Orkney Isles. 



Squali (Sharks and Dog-fishes). 



In the suborder Squali are included all the Sharks and Dog- 

 fishes except the few embraced in the former suborder, the 

 Notidani. There are two dorsal fins instead of one and the gill- 

 clefts are five in number on each side. These fishes differ from 

 those of the following suborder, the Kaii (Wall-case 3), in the 

 gill-clefts being laterally placed, and in the body being of the 

 usual Shark type, without any great enlargement of the pectoral 

 fins or flattening of the body. The Rhinida3 show a tendency to 

 modification in these directions, although they do not necessarily 

 lead on to the Rays. It is convenient to divide the suborder into 

 two groups, which may be designated Group A and Group B. 



