526 SHARKS AND RAYS. 



nictitating membrane to the eye ; and also by the solid structure of the fully 

 formed teeth, which are pointed, and in most of the genera relatively large. In 

 addition to these features, it may be noted that the gill-openings are generally 

 wide, and the spiracles either minute or wanting. This family dates from the 

 period of the Chalk, where there occur remains of species some of which are 

 referable to genera still existing, such as the porbeagles, while others indicate 

 extinct generic type. The fox-sharks and the gigantic Carcharodon are, however, 

 unknown before the Tertiary period. 



The shark (Lamina cornubica) commonly known to the British 



P0I*t)6£L£rl.6S 



fishermen as the porbeagle — a word supposed to be derived from 



its porpoise-like appearance and active predatory habits — is the type of a genus 



containing three existing species, and characterised by the small size of the second 



dorsal and anal fin, and the presence of a pit at the root of the caudal fin — of which 



the lower lobe is much developed — and also of a keel along the sides of the tail. 



The teeth are narrow and slender, with one or two pairs of small accessory cones 



at their bases ; the edges of the main cone being smooth. The common porbeagle 



wanders all over the North Atlantic, and has also been taken in Japan ; it does 



not commonly exceed 10 feet in length, and its colour is dull grey above and 



whitish beneath. Its food chiefly consists of fishes, which are apparently 



swallowed whole ; the lancet-like teeth of this shark being apparently more 



adapted for seizing and holding than for tearing prey. The porbeagle is stated to 



be a viviparous species. 



Kondeietis The most formidable of all the existing members of the group is 



Shark. £he gigantic Rondeleti's shark (Carcharodon rondeletii), distinguished 



from the porbeagles by the great size of the broadly triangular teeth, which have 



strongly serrated edges, and may possess basal cusps. The existing species, which 



is a purely pelagic creature ranging over all the warmer seas, is known to attain 



a length of 40 feet, one of the teeth of a specimen of 36 feet in length measuring 



2 inches along the edge of the crown, and If inches across the base. Similar 



teeth are found in the Crag deposits of Suffolk, and are referred to the existing 



species ; but from these same beds, and also from the bottom of the Pacific, 



between Polynesia and Australia, there are obtained other teeth of much larger 



dimensions, some of them measuring upwards of 5 inches along the edge and 



4 inches in basal depth. These teeth evidently indicate sharks beside which the 



existing form is a comparative dwarf ; and it is not a little remarkable that the 



specimens dredged from the bed of the Pacific indicate that these giants must in 



all probability have survived to a comparatively recent date. Observations are 



still required as to the mode of life and breeding-habits of Rondeleti's shark. 



Two other species of large sharks constitute the genus Odontaspis. With teeth 



almost indistinguishable from those of the porbeagles, these species differ by the 



second dorsal and anal fins being nearly as large as the first dorsal, and the 



absence of a pit at the root of the caudal fin, and of a keel on the sides of the tail. 



Another species not uncommonly met with in British waters is 

 Fox-Shark r ^ 



the fox-shark or thresher (Alopecias vvdpes), the sole representative 



of its genus, and easily recognised by the inordinate length of the upper lobe of 



its tail-fin, from which it derives its name. Growing: to a length of 15 feet, of 



