5 2 <5 
SHARKS AND KAYS. 
Porbeagles. 
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 Carchctrodon 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 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 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. 
Another species not uncommonly met with in British waters is 
the 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 
Fox-Shark. 
