A MONOGRAPH OF THE SEA-SNAKES (HYDROPHIINsE). 175 



occur in Enhydris hardwickii , where it always runs to the first supralabial, and in rare 

 examples of Distira ornata and Enhydrina valakadyn where it takes a similar course. 

 The tendency for these shields to split is seen not infrequently in Acalyptus peroni, 

 both Thalassophis , Enhydris hardwickii , Enhydrina valakadyn, Distira nigrocincta , 

 D. viperina and D. ornata. (See figs. 40, 55 and 59). The condition is too inconstant 

 to offer any help in classification. 



Internasals. — These shields are present in the genera Platurus only, where there 

 are two, except in P. schistorhynchus. In this species there are two rows of shields, 

 one anteriorly and usually two behind. (See fig. 1). 



Praefrontals. — These are present in all the species and consist of a pair with a 

 few exceptions, which occur in the genera Platurus, Emydocephalus and Aipysurus and 

 Thalassophis. In Platurus schistorhynchus there are three, and in Emydocephalus ijimce 

 and in Aipysurus there may be four, but the condition in the two last is an inconstant 

 one, the usual prefrontals seen in other forms being subdivided on one or both sides in 

 some specimens only, so that the number of these shields does not aid classification. 

 In Thalassophis annandalei there are many. 



Normally in all the species the fellows of the pair are in contact, but in rare 

 individuals of certain species the frontal is projected so far forward as to completely 

 separate the fellows. I have seen this most frequently in Distira viperina, but also 

 in D. jerdoni and some other species. It occurs in the type-specimen of Jan's fron- 

 talis, in a specimen in the British Museum referred by Mr. Boulenger to frontalis 

 (Jan), but which I take to be ornata (Gray), and in the type-specimen of brookii 

 (Giinther). In the latter case the specimen is a gravid female, and the condition is not 

 inherited by her unborn young (Boulenger Catalogue, vol. iii, p. 283). The relationship 

 of the prefrontals with the supralabials is, I consider, of great importance. I 

 find the relationship invariable in most genera, but in individuals of Enhydrina and 

 Astrotia it is subject to some variation, and also in individuals of some species of 

 Distira. In order to justify this assertion, I may remark that in some examples the 

 relationship differs on the two sides, and it is usually very obvious when attention is 

 paid to other characters which to consider the abnormal side. The unilateral abnormal- 

 ity naturally prepares one for the still rarer exception, in which the abnormality is 

 bilateral. This remark applies with equal force to many other abnormalities alluded to 

 as such in the headshields of individuals. In many genera these shields touch no supra- 

 Labial, as for instance Platurus, Aipysurus , Emydocephalus , Acalyptus , Thalassophis and 

 a few species of Distira, notably jerdoni, nigrocincta and viperina. In nearly all the other 

 species the contact is with the second supralabial. In Hydrelaps darwiniensis it touches 

 the second and third, as it does also in some specimens of Astrotia stokesi and Hydrus 

 platurus. In Distira cantoris it touches the third supralabial only (rarely the second 

 also) . The contact of this shield with the eye is unique in Hydrelaps darwiniensis. (See 

 fig. 8) . I have, however, seen this as an aberrant feature in D. obscura owing to a conflu- 

 ence of the praeocular with the praefrontal. In one specimen it occurs on one side 

 only, in another on both sides, and in one example of D. jerdoni on both sides. The 



