A MONOGRAPH OF THE SEA-SNAKES (HYDROPAHIIN). 179 
Distiva jerdoni (fig. 58) there are three only, but in all the other forms four are pres- 
ent excepting Aipysurus australis, where they are too ill developed to deserve the 
name. 
The first on each side meet behind the mental (except in a few abnormal indi- 
viduals of a few species) and form a suture, the length of which compared with that 
between the anterior sublinguals has some importance. In Diéstiva cantoris, D. graci- 
lis, D. obscura and D. fasciata, etc , this suture is much longer than that between the 
anterior sublinguals, but in almost all the other species it is little longer and often 
shorter. The last infralabial is peculiar in Aipysurus eydouxit, Acalyptus peront, 
Platurus schistorhynchus, Distira cantoris, D. gracilis and D. jerdont, in that it touches 
but two scales behind. In all the other species it touches three or four. 
MarGINALs.—I apply this term to certain small cuneate scales which are, in many 
species, intercalated between the infralabials at the labial margin. They are very 
distinctive in form, and not to be confused with divided infralabials, the outer parts of 
which are not cuneate in outline. Examples of divided infralabials are shown in 
figs. 12B where the third is divided on the left side, in 24C where the first is 
so distinguished on the left side, and in 59C where the fourth is divided on both sides. 
Their constancy though apparently complete in many species is less so in others, 
thereby detracting somewhat from their value; still they are fully as important as 
many other characters upon which one has to rely in separating species, especially 
those of the genus Distiva. ‘They are absent in D. gracilis, D. cantoris and D. jerdont, 
there being no exceptions in the large series of each that I have examined. Similarly, 
one or more are present in the large series of viperina (20), cerulescens (29), fasciata 
(34), torquata (29), without any exception. In most of the other species of Distira 
the constancy is not so complete though very striking. The constancy in the number 
of these little shields when present is not so striking, for though a very large number 
of individuals in many species have but one, and that wedged between the third and 
fourth infralabials, there is a tendency for more to be present, and they may succeed 
the second infralabial. In /fasciata, for instance, five specimens out of 38 have two 
marginals occurring after the second or third infralabials on one or both sides, in all the 
rest there is but one, and that after the third. Theconstancy in number and disposi- 
tion though not complete is as striking in obscura, cerulescens, etc. 
SUBLINGUALS.—The ‘‘Chin shields’’ of other authors. ‘There are usually two 
pairs, the fellows of each in contact with one another. In Astrotia both pairs are 
absent. In Hydrus, Enhydris and Enhydrina they are poorly developed, especially 
the posterior, if they can be said to be present at all; and the anterior pair frequently 
present though small, has the fellows widely separated. In Distiva major, D. ornata 
and D. cerulescens they may also besmall, but the anterior pair is very generally 
present and the fellows in contact; the posterior, when recognisable, are usually well 
separated by small scales. 
The contact or separation of the posterior pair when developed, though showing 
great constancy in some species, manifests frequent variation in individuals of other 
