THE PORBEAGLE GROUP 2901 



the smooth hound generally makes its appearance during the summer in pursuit of 

 the shoals of pilchard and herring. Several other genera of this family must be 

 passed over without notice. 



THE PORBEAGLE GROUP Family LAMNID^E 



Agreeing with the typical sharks in the position of the two spineless dorsal 

 fins, the members of the present family may be distinguished by the absence of a 

 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 fish- 

 ermen as the porbeagle a word supposed to be derived from its por- 

 poise-like appearance and active predatory habits is a type of a genus containing 

 three existing species, and characterized 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 ten feet in length, and its color is dull gray 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. 



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

 the gigantic Rondeleti's shark (Carcharodon rondeletii), distinguished 

 from the porbeagles by the great size of the broad 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 forty feet, one of the teeth of a specimen of thirty-six 

 feet in length measuring two inches along the edge of the crown, and one and 

 three-fourths 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 upward of five 

 inches along the edge and four inches in basal depth. These teeth evidently indi- 

 cate 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 



