Eye without nictitating membrane. Anal fin present. Two dorsal fins; the first opposite to the space between pectoral and ventral fins, without spine in front. Nostrils not confluent with the mouth which is inferior. Spiracles absent or minute.
All the fishes of this family attain to a very large size, and are pelagic. But little is known of their reproduction. The first appearance of this family is indicated by Carcharopsis, a genus from carboniferous formations, the teeth of which differ from those of Carcharodon only by having a broad fold at the base. In the chalk and tertiary formations almost all the existing genera are represented; and, besides, Oxytes, Sphenodus, Gomphodus, and Ancistrodon, which are known from teeth only, have been considered generically distinct from the living Porbeagles.
Lamna (Oxyrhina).—The second dorsal and anal are very small. A pit at the root of the caudal, which has the lower lobe much developed. Side of the tail with a prominent longitudinal keel. Mouth wide. Teeth large, lanceolate, not serrated, sometimes with additional basal cusps. On each side of the upper jaw, at some distance from the symphysis, there is one or two teeth conspicuously smaller than the others. Gill-openings very wide. Spiracles minute.
Fig. 113.—Upper and lower tooth of Lamna.
Of the “Porbeagles,” three species have been described, of which the one occurring in the North Atlantic, and frequently straying to the British coasts (L. cornubica), is best known. It attains to a length of ten feet, and feeds chiefly on fishes; its lanceolate teeth are not adapted for cutting, but rather for seizing and holding its prey, which it appears to swallow whole. According to Pennant it is viviparous; only two embryoes were found in the female which came under his observation. Haast has found this species also off the coast of New Zealand.
Carcharodon.—The second dorsal and anal are very small. Pit at the root of the caudal, which has the lower lobe well developed. Side of the tail with a prominent longitudinal keel. Mouth wide. Teeth large, flat, erect, regularly triangular, serrated. On each side of the upper jaw, at some distance from the symphysis, there is one or two teeth conspicuously smaller than the others. Gill-openings wide.
One species only is known (C. rondeletii), which is the most formidable of all Sharks. It is strictly pelagic; and appears to occur in all tropical and sub-tropical seas. It is known to attain to a length of 40 feet. The tooth figured here, of the natural size, is taken from a jaw 20 inches wide in its transverse diameter (inside measure), each half of the mandible measuring 22 inches.[36] The whole length of the fish was 36½ feet.
Carcharodon teeth are of very common occurrence in various tertiary strata, and have been referred to several species, affording ample evidence that this type was much more numerously represented in that geological epoch than in the recent fauna. Some individuals attained to an immense size, as we may judge from teeth found in the Crag, which are 4 inches wide at the base, and 5 inches long, measured along their lateral margin. The naturalists of the “Challenger” expedition have made the highly interesting discovery that teeth of similar size are of common occurrence in the ooze of the Pacific, between Polynesia and the west coast of America. As we have no record of living individuals of that bulk having been observed, the gigantic species to which these teeth belonged must have become extinct within a comparatively recent period. Nothing is known of the anatomy, habits, and reproduction of the surviving species, and no opportunity should be lost of obtaining information on this Shark.
