The number of species at present known is considerable; they are divided into _ten_ genera. We shall confine ourselves here to mentioning the principal diagnostic characters of these genera, and to describing the most common species. [Illustration: FIG. 77.--SKULL OF _Hydrus platurus_. (After G. A. Boulenger, _op. cit._)] (1) =Hydrus.= (Fig. 77.) Maxillaries longer than the ectopterygoids, not extending forwards as far as the palatines; poison-fangs grooved, rather short, followed, after an interspace, by 7 or 8 solid, backwardly curved teeth. Snout long, bearing the nostrils on its upper surface; head-shields large, nasals in contact with each other. Body rather short; scales hexagonal or squarish, juxtaposed; no distinct ventral scales. [Illustration: FIG. 78.--_Hydrus platurus_ (syn. _Pelamis bicolor_). (After Krefft.)] The principal species of this genus is _H. platurus_ (syn. _Pelamis bicolor_, fig. 78). Coloration black or brown and yellow, with very variable markings. Total length, 700 millimetres; tail 80. _Habitat_: Indian Ocean, Tropical and Sub-tropical Pacific. (2) =Thalassophis.= Poison-fangs followed by 5 small teeth. Snout short; nostrils superior, horizontal, between two nasal shields and an internasal; frontal and parietal shields large; præocular present. Body rather elongate; scales hexagonal, juxtaposed; no distinct ventral scales. _T. anomalus._--Body with dark annuli, wider on the back. Total length, 810 millimetres; tail 84. _Habitat_: Java. (3) =Acalyptophis.= Maxillaries longer than the ectopterygoids; frontal and parietal shields broken up into scales. Body rather elongate; scales subimbricate; no distinct ventrals. _A. peronii._--Greyish or pale olive, with dark cross-bands; belly whitish. Total length, 890 millimetres; tail 115. _Habitat_: Western Tropical Pacific. (4) =Hydrelaps.= Snout short; 6 teeth behind the poison-fangs; nostril in a single nasal shield; head-shields large. Body feebly compressed; scales imbricate; ventral scales small, but well developed. _H. darwiniensis._--Body with yellowish-white and blackish annuli, the black rings narrower on the belly; head dark olive spotted with black. Total length, 435 millimetres; tail 43. _Habitat_: North Australia. (5) =Hydrophis.= (FIG. 79.) [Illustration: FIG. 79.--_Hydrophis coronatus._ (After Sir Joseph Fayrer.)] Poison-fangs large, followed by a series of 7-18 solid teeth. Head small; nostrils on the upper surface of the snout, pierced in a single nasal shield; head-shields large; præocular present. Body long, often very slender anteriorly; scales on the anterior part of the body imbricate, rectangular, keeled or tubercular; ventrals more or less distinct, very small. A considerable number of (at least 22) species of HYDROPHIS are known. Those most frequently met with are the following:--