A MONOGRAPH OF THE SEHA-SNAKES (HYDROPHIINA). 175 
occur in Enhydris hardwickit, where it always runs to the first supralabial, and in rare 
examples of Distiva ornata and Enhydrina valakadyn where it takes a similar course. 
The tendency for these shields to split is seen not infrequently in Acalyptus peronz, 
both Thalassophis, Enhydris hardwickii, Enhydrina valakadyn, Distiva 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. I). 
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 ime 
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 annandale: 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 Distiva 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 brooki 
(Giinther). In the latter case the specimen is a gravid female, and the condition is not 
inherited by her unborn young (Boulenger Catalogue, vol. ili, 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 
Distiva. 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 considerthe 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, asforinstance Platurus, Aipysurus, Emydocephalus, Acalyptus, Thalassophis and 
a few species of Distiva, notably jerdont, 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 stokest 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 uniquein Hydrelaps darwiniensis. (See 
fig. 8). I have, however, seen this as an aberrant feature in D. obscura owing toa 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 
