HYDROPHID.E. 353 



are placed quite at the top of the snout, as in crocodiles and in freshwater snakes, so that 

 they are enabled to breathe whilst the entire body and the greater part of the head are 

 immersed in the water. These openings ai'e small and subcrescentic, and provided with 

 a valve interiorly, which is opened during respiration, and closed when the animal dives. 

 They have very capacious lungs, extending backwards to the anus, and consequently all their 

 ribs are employed in performing the respiratory function ; by retaining a portion of the air 

 in these extensive lungs, they are enabled to float on the surface of the water without the 

 slightest effcjrt. 



The " scales " of the Sea-snakes are frequently very diff"erent from those of other snakes ; 

 they overlap one another in only a few species (Platums, Hz/drojiMs stokesii and helcheri), in 

 others they are but little imbricate and rounded behind, and, agam, in others they are of a 

 subquadrangular or hexagonal form, placed side by side, like little shields ; the less imbri- 

 cate they are, the more they have lost the polished surface which we find in other snakes, 

 and are soft, tubercular, sometimes porous. The form and the arrangement of the scales 

 aff'ord good specific characters, but it is necessary to observe that they generally difi^er in size, 

 arrangement, and form in the dift'erent parts of the body. In my descriptions I have always 

 counted the series of scales on the neck — that is, at a distance from the head about equal to 

 its length, having found that the numbers at that place are least subject to variation in 

 dift'erent individuals ; whilst I have taken the characters of the form or arrangement of the 

 scales from those in or behind the middle of the body, the scales on the neck being narrow 

 and more or less imbricate in almost all the species. 



The shields of the head differ so much in their arrangement from those of other snakes, 

 that a snake may be recognized as a marine species by an inspection of the head only. 

 The large nasal shields occupy the upper anterior part of the snout and are generally con- 

 tiguous, replacing the anterior frontals which are absent ; the single pair of small frontals 

 are homologous with the posterior frontals of other snakes. There is a vertical, a pair of 

 supraciliaries and another of occipitals, one ocular, and one or two postoculars ; the number 

 of the latter is rather constant in the same species. Loreal none. The labials are somewhat 

 irregularly arranged, frequently subdivided, especially the posterior ; in most of the species, 

 small pieces, nearest to the labial margin, are detached from the lower labials. There is a 

 triangular mental shield in front of the lovt-er jaw, behind which the first pair of lower labials 

 form a suture together ; one or two pairs of chin-shields follow. Several Sea-snakes are dis- 

 tinguished by having some or all of the head-shields broken up into smaller irregular pieces, 

 whilst Flafurus difiers from all others in an arrangement of the sliields which is extremely 

 similar to that in the Elapidce. The Sea-snakes shed their skin very frequently, and the 

 skin peels ofi" in pieces as in the lizards, and not as in tlie freshwater serpents, in which the 

 integuments come off entire. 



Several species are remarkable for the extremely slender and prolonged anterior part of 

 the body, for which we use the term " neck," and which terminates in a very small head. 

 These snakes can hardly form a separate genus, as we find a most complete transition from 

 them to the forms with thick and short body. The extreme forms must diff'er considerably 

 in their habits, but no observations on this point are on record. 



The eye is small, with round pupil, which is so much contracted by the light when the 

 snake is taken out of the water, that the animal becomes blinded and is unable to hit any 



