in “Annal. Mus. Civ. Geneva,” 1874 and 1878. THIRD FAMILY--RHINODONTIDÆ. _No nictitating membrane. Anal fin present. Two dorsal fins, the first nearly opposite to the ventrals, without spine in front. Mouth and nostril near the extremity of the snout._ This small family comprises one species only, _Rhinodon typicus_, a gigantic Shark, which is known to exceed a length of fifty feet, but is stated to attain that of seventy. It does not appear to be rare in the western parts of the Indian Ocean, and possibly occurs also in the Pacific. It is one of the most interesting forms, not unlike the Basking Shark of the Northern Seas, having gill-rakers like that species; but very little is known of its structure and mode of life. It is perfectly harmless, its teeth being extremely small and numerous, placed in broad bands; it has been stated to feed on tang, an observation which requires confirmation. The snout is very broad, short, and flat; the eyes are very small. A pit at the root of the caudal fin which has the lower lobe well developed; side of the tail with a keel. A characteristic figure of this fish has been given by A. Smith in his “Illustrations of the Zoology of South Africa,” Plate 26, from a specimen which came ashore at the Cape of Good Hope. [Illustration: Fig. 115.--Dentition of Notidanus indicus. _a_, teeth in function; _b_, teeth in reserve; _u_, upper, and _l_, lower, tooth, of natural size.] FOURTH FAMILY--NOTIDANIDÆ. _No nictitating membrane. One dorsal fin only, without spine, opposite to the anal._ NOTIDANUS.--Dentition unequal in the jaws: in the upper jaw one or two pairs of awl-shaped teeth, the following six being broader, and provided with several cusps, one of which is much the strongest. Lower jaw with six large comb-like teeth on each side, beside the smaller posterior teeth. Spiracles small, on the side of the neck. No pit at the root of the caudal fin. Gill-openings wide, six in number in _Hexanchus_, seven in _Heptanchus_. Four species are known, distributed over nearly all the tropical and sub-tropical seas; they attain to a length of about fifteen feet. Fossil teeth belonging to this type have been found in Jurassic and later formations (_Notidanus_ and _Aellopos_). FIFTH FAMILY--SCYLLIIDÆ. _Two dorsal fins, without spine: the first above or behind the ventrals; anal fin present. No nictitating membrane. Spiracle always distinct. Mouth inferior. Teeth small, several series generally being in function._ SCYLLIUM.--The origin of the anal fin is always in advance of that of the second dorsal. Nasal cavity separate from the mouth. Teeth small, with a middle longer cusp, and generally one or two small lateral cusps arranged in numerous series. Eggs similar to those of the Rays (Fig. 79, p. 167).