150 SUB-CLASS ELASMOBRANCHII. 



Fam. 2. Notidanidae. Mouth sub-inferior ; nostrik on lower side, 

 nearer snout than mouth ; dentition unequal in the two jaws ; in the upper 

 jaw one or two pairs of awl-shaped teeth, the next six teeth broader and 

 each provided with several cusps, one of which is the strongest ; lower 

 jaw with six large, comb-like teeth on each side, beside the smaller pos- 

 terior teeth ; viviparous ; sometimes reach a large size. The palato- 

 quadrate articulates with the postorbital process of the skull and the hyo- 

 mandibular is comparatively slender ; each segment of the vertebral 

 column of the middle part of the trunk region carries two neural arches 

 and corresponds to two pairs of spinal nerves (p. 126); the pseudobranch 

 is very large and has several well-developed laminae. Temperate and 

 warm seas. Hexanchus Raf., with six pairs gill-apertures ; vertebrae 

 without calcification ; H. griseus Gmelin, 8-26 feet, B't^editerranean, 

 W. coast Scotland ; Heptanchus Raf., with seven pairs gill- apertures, 

 vertebrae asterospondylous. 



Sub-order 2. SQUALL 



Vertebral column well segmented, vertebrae amphicoelous 

 with a double cone of calcified cartilage, outside which and 

 springing from it there may be radiating calcareous lamellae 

 (asterospondylous) or additional concentric calcified rings (cyclo- 

 spondylous) ; with two dorsal fins, and with or without anal fins. 

 With five gill apertures laterally placed, spiracle present or 

 absent, never large. The palatoquadrate is not articulated 

 directly to the skull except in Cestraciontidae. 



This sub-order includes the great body of living sharks, and has 

 existed since the Silurian period. Some of the living genera have 

 existed since early times ; e.g. Cestracion, Upper Jurassic ; 

 ScylUum, and Scapanorhynchus {Mitsukurina) Cretaceous ; 

 Pristiurus, Upper Jurassic. Most of them are active predatory 

 creatures, and some attain a considerable size. The largest 

 are however harmless creatures, which like the whalebone 

 whales exist on small marine organisms which are detained on 

 their prolonged gill-rakers {Selache, Rhinodon). 



Fam. 3. Cestraciontidae. Bull-head sharks ; asterospondylous ; the 

 palato-quadrate articulates by an extensive surface with the preorbital 

 region of the skull ; two dorsal fins with spines, the first dorsal opposite 

 the space between pectorals and pelvics, the second in advance of the 

 anal ; upper lip divided into seven lobes, the lower with fold ; spiracle 

 small, below posterior part of eye ; without nictitating membrane ; den- 

 tition similar in both jaws, viz. small obtuse teeth in front, pointed and 

 provided in young individuals with three to five cusps ; lateral teeth 

 large, pad-like, twice as long as broad ; Pacific and East Indian Archi- 

 pelago ; size small ; oviparous, egg-case spirally twisted. Cestracioti 

 Cuv. {Heterodontus Blainv.), C. phillipi Blainv., Port- Jackson shark. 



