such as would be produced by a serrated spine, is not unlikely to assume a serious character, under the influence of a tropical climate.  The species figured on the last page is brownish-olive on the upper surface, with numerous greenish-white round spots, darkening towards the edges.  The anterior annulations of the tail are black and white, the posterior entirely black.  Its mouth is transverse and paved with a band of flattened teeth calculated to crush the hard shells of the animals on which it feeds.  It moves slowly along the bottom in search of its food, which consists of crustacea and mollusca, and seems to be unable to catch fishes or other quickly moving animals.  Specimens have been taken near Ceylon, of six feet in width.  Like most deep-sea fishes, the ray has a wide geographical range, and occurs not only in all the Indian Ocean, but also in the tropical tracts of the Atlantic.

[Illustration:  THE RAY (AETOBATES NARINARI).]

[Footnote 1:  Raja narinari, Bl.  Schn. p. 361. Aetobates narinari, Muell. und Henle., Plagiost. p. 179.]

Another armed fish, renowned since the times of AElian and Pliny for its courage in attacking the whale, and even a ship, is the sword-fish (Xiphias gladius).[1] Like the thunny and bonito, it is an inhabitant of the deeper seas, and, though known in the Mediterranean, is chiefly confined to the tropics.  The dangerous weapon with which nature has equipped it is formed by the prolongation and intertexture of the bones of the upper jaw into an exceedingly compact cylindrical protuberance, somewhat flattened at the base, but tapering to a sharp point.  In strange inconsistence with its possession of so formidable an armature, the general disposition of the sword-fish is represented to be gentle and inoffensive; and although the fact of its assaults upon the whale has been incontestably established, yet the motive for such conflicts, and the causes of its enmity, are beyond conjecture.  Competition for food is out of the question, as the Xiphias can find its own supplies without rivalry on the part of its gigantic antagonist; and as to converting the whale itself into food, the sword-fish, from the construction of its mouth and the small size of its teeth, is quite incapable of feeding on animals of such dimensions.

[Footnote 1:  AELIAN tells a story of a ship in the Black Sea, the bottom of which was penetrated by the sword of a Xiphias (L. xiv. c. 23); and PLINY (L. xxxii. c. 8) speaks of a similar accident on the coast of Mauritania.  In the British Museum there is a specimen of a plank of oak, pierced by a sword-fish, and still retaining the broken weapon.]

In the seas around Ceylon sword-fishes sometimes attain to the length of twenty feet, and are distinguished by the unusual height of the dorsal fin.  Those both of the Atlantic and Mediterranean possess this fin in its full proportions, only during the earlier stages of their growth.  Its dimensions even then are much smaller than in the Indian species; and it is a curious fact that it gradually decreases as the fish approaches to maturity; whereas in the seas around Ceylon, it retains its full size throughout the entire period of life.  They raise it above the water, whilst dashing along the surface in their rapid course; and there is no reason to doubt that it occasionally acts as a sail.