5 1 8 PISCES— SUB- CLASS IV.—ELASMpBRANCHIT. 



The dog-Bshes and allied members of this family differ from the foregoing 

 sharks with the mouth terminal by the backward position of the second 

 dorsal fin which occupies a position immediately over, or 

 Family Scyl- behind the line of the pelvic pair. The teeth, of which 

 liidm. several series are generally in use, are of small size, the eye 



lacks the nictitating membrane, and spiracles are present on 

 the head. The dog-tishes, of which there are two species from Britisli 

 waters, form the genus ScylUum ; most of the species having their skins 

 elegantly spotted. Much larger is the Indian zebra-shark (Stegostoma), 

 growing to some fifteen feet in length, and taking its name from its zebra or 

 tiger-like coloration. Mention must also be made of the sharks of the genus 

 Grossorhinns, from Japan and Australia, not only on account of the circum- 

 stance that they are deep-water forms, but likewise from the presence of 

 leaf-like outgrowths of the skin of the sides of the head ; such appendages 

 beiug probably for the purpose of attracting prey within reach of the 

 jaws. 



The Port Jackson shark, together with three allied species, form the genus 



Cestracion (or more properly, Cestracium ^), which now alone constitutes a 



family easily recognised by the peculiar character of tlie 



Family dentition, and likewise by the presence of a spine in the 



Ceslraciid'V. front portion of each dorsal fin. Whereas in the forepart of 



both jaws the numerous rows of teeth are sharp and cusped, 



on the sides and hinder region they form a pavement-like structure, arranged 



in oblique rows ; the teeth in some of these rows being much larger than 



in the others. The 

 mouth is situated at 

 the extremity of the 

 muzzle, and the eye 

 is unprovided with a 

 nicti tating membrane. 

 These sharks, which do 

 not grow to more than 

 about five feet in 

 length, are widely dis- 

 tributedin thewarmer 

 seas. Although we 

 Fig. 37.— Port Jackson Shaek. are still very ignorant 



. . as to their life-his- 



tory, It IS known that their food is principally composed of shell-fish, 

 and also that their eggs are unlike those of other members of the 

 order, their enveloping membranes being twisted into the form of a 

 screw. 



The two genera of sharks constituting this small sub-family stand apart 



from the other members of the sub-order in regard to the maimer in which 



the jaws are articulated to the cranium, while externally 



FamUy they are readily distinguished by the number of gill-clefts 



Notidanidoi. being increased from the normal five to either six or seven, 



, '^"d also by the single dorsal fin. There is no spine to the 



latter and the teeth are cusped, and form several series. The typical genus 



iVotvrf.nDM includes f(jur species, characterised by the simple gill-slits and 



the complicated structure of the teeth, which consist of an elongated base 



1 The name of the family is commonly given as Ccilradontida, but it should be as written liere. 



