HTDKOPHIINiE. 265 



hypapophyses very much developed. Hypapophyses not developed 

 throughout the vertebral column. 



Body more or less compressed ; eye small or very small, with 

 round pupil. Eostral shield with two notches in the oral border, 

 only the cleft portion of the tongue being protrusible. 



Marine (with the exception of one species of Distira, confined to a 

 freshwater lake in Luzon), entering tidal streams. With the ex- 

 ception of Platurus, specimens of which have repeatedly been found 

 at some distance from water, these snakes are exclusively aquatic ; 

 all are viviparous. They are confined to the Indian and Pacific 

 Oceans, one species (Hydrus platurus) being widely distributed, but 

 do not seem to occur on the East Coast of Africa. The habitat of 

 the greater number extends from the Persian Gulf to the Western 

 Tropical Pacific. 



Synopsis of the Genera. 



I. Maxillary not extending forwards beyond the palatine ; ventral 

 shields small or absent ; nostrils on the upper surface of the 

 snout. 



A. Symphysial shield triangxdar, not concealed in a mental groove. 



1. Maxillary longer than the lower aspect of the ectopterygoid, 



with 4 to 18 small teeth in addition to the poison-fangs. 



a. No ventral shields. 

 Nostril in the nasal shield 204. Hydrus, p. 266. 



Nostril between two nasals and an internasal. 



205. Thalassophis, p. 268. 



Nostril between two nasals and the prsefrontal. 



206. Acalyptophis, p. 269. 



6. Ventral shields distinct, at least on the anterior part of 

 the body. 



a. No prsBocular 207. Hydrelaps, p. 270. 



|3. Praeocular present. 



Small maxillary teeth not grooved ; body often very slender 

 anteriorly 208. Hydrophis, p. 271. 



AU the maxillary teeth grooved 209. Distira, p. 285. 



2. Maxillary not longer than the ectopterygoid, with 2 to 5 



small grooved teeth in addition to the poison-fangs. 



210. Enhydris, p. 300. 



B. Symphvsial shield narrow, partly concealed in a deep groove 



in the chin 211. Enhydrina, p. 302. 



