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 (Lamna cornubica) commonly known to the British 



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. 



Rondeieti's The most formidable of all the existing members of the group is 



Shark. the gigantic Rondeieti'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 froih 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 Rondeieti'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. 

 FOX s&ark Another species not uncommonly met with in British waters is 



Lhe fox-shark or thresher (Alopecias vulpes), 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 



