Seven species from tropical and sub-tropical seas are known; of _Rhinoptera polyodon_ nothing is known except the jaws; and as its dentition is very peculiar, no opportunity should be lost of obtaining and preserving entire animals. Teeth very similar to those of existing species, and described as _Zygobatis_, occur in the Norwich Crag and in Miocene formations of Switzerland. DICEROBATIS (CEPHALOPTERA).--Cephalic appendages pointing straight forwards or inwards. Nostrils widely separated from each other. Mouth inferior, wide. Both jaws with very numerous and very small flat or tubercular teeth. Tail very slender, with a dorsal fin between the ventrals, and with or without a serrated spine. CERATOPTERA.--Cephalic appendages pointing forwards or inwards. Mouth anterior; wide. Teeth in the lower jaw only, very small. Tail very slender, with a dorsal fin between the ventrals and without spine. [Illustration: Fig. 134.--Dicerobatis draco, from Misol.] The species of these two last genera are not yet well distinguished; about five of _Dicerobatis_ and two of _Ceratoptera_ are known from tropical and temperate seas, but their occurrence in the latter is rather sporadic. Some of them, if not all, attain an enormous size. One mentioned by Risso, taken off Messina, weighed 1250 pounds. Several observers speak of having seen them in pairs, the male being usually the smaller. Of a pair mentioned by Risso the female was first taken, and the male remained hovering about the boat for three days, and was afterwards found floating dead on the surface. Still larger individuals, but of uncertain species, are mentioned by Lacépède, who says that one taken at Barbadoes required seven yoke of oxen to draw it. A sketch of another, which was said to be twenty feet long, was sent to Lacépède; and Sonnini speaks of one which appeared to him to be longer and wider than the ship in which he was sailing. A fœtus taken from the uterus of the mother captured at Jamaica, and preserved in the British Museum, is five feet broad, and weighed twenty pounds. The mother measured fifteen feet in width as well as in length, and was between three and four feet thick. The capture of “Devil-fishes” of such large size is attended with danger, as they not rarely attack and capsize the boat. They are said to be especially dangerous when they accompany their young, of which they bring forth one only at a time. SECOND SUB-ORDER--HOLOCEPHALA. _One external gill-opening only, covered by a fold of the skin, which encloses a rudimentary cartilaginous gill-cover; four branchial clefts within the gill-cavity. The maxillary and palatal apparatus coalescent with the skull._