172 MAJOR F. WALL, I.M.S., C.M.Z.S. 
Numbers.—These may be the same or proximately the same (within 2) in the 
whole body length, or relatively more numerous posteriorly than anteriorly, and this is 
of great importance in separating genera. In Platurus, Emydocephalus and Arpysurus 
the rows are the same throughout, or vary but slightly. The same is a noticeable 
feature of Distira jerdoni, and one of my strongest reasons for believing that this species 
should occupy a place apart under a genus to itself. The degree to which the rows 
posteriorly may exceed those anteriorly in the same specimen varies considerably 
in different genera and species, but the range of variation to be met with in these two 
counts in individuals of the same species is such as to detract considerably from the 
assistance to be derived from this condition, especially in closely alliedforms. In order 
to obtain the best results, I count these scales in three situations, v2z., anteriorly, 7.e., 
two headslengths behind the head, in mid body, and posteriorly, 7.e., two headslengths 
before the anus. ‘Theterms “ anterior”’ and ‘‘ posterior’’ used throughout this mono- 
graph are therefore precise. In Distiva spiralis they may be from two to nine more 
posteriorly than anteriorly, in D. fasciata 10 to 22 more, and between these extremes 
every degree is to be met with in various other species. 
The actual numbers of rows are very important in another way, assisting the dis- 
tinction of genera and species. They are fewest in Emydocephalus (17 to 19), Atpysurus 
(except /e@vis) (17 to 19), and Distiva jerdom (19); most numerous in Thalassophis 
annandalet (go to 100), Enhydrina (50 to 70), Hydrus (45 to 62), and Astrotia (48 to 
59). In Distiva gracilis they are very few anteriorly (17 to 21), and rather numerous 
posteriorly (27 to 35). Again the numbers of rows may be very constant in individuals 
of the same species, or the reverse, a condition influencing generic and specific classi- 
fication. In Platurus, Emydocephalus and Atpysurus, individuals have a like number of 
rows, or range within two of the normal; on the other hand in certain species the rows 
counted at the same site vary in individuals very considerably, notably in Enhydrina 
from 50 to 70, Hydrus 45 to 62, Distiva fasciata 37 to 51. Every degree of variation 
may be met with between these extremes. 
Imbrication.—The costals may be imbricate, subimbricate, or juxtaposed, and this 
condition is of great importance generically, as well as assisting the separation of 
certain species in the genus Distiva. In the genera Platurus, Emydocephalus, Aipysurus 
and Astrotia imbrication is pronounced in the whole body length, also in Distiva jerdont. 
In Hydrus and Enhydris the costals are juxtaposed throughout, and in certain species, 
notably Distiva cantoris and D. gracilis, these scales are imbricate anteriorly, juxtaposed 
posteriorly. In certain species this condition is subject to variation, notably in Distira 
fasciata, D. torquata, D. ornata, D. cerulescens, etc., specimens being met with in which 
these scales are juxtaposed, and others in which they are distinctly imbricate or sub- 
imbricate posteriorly. ‘This fact shows that, important as this condition is, it cannot 
be completely relied upon, and one is to expect a similar aberration in individuals of 
other allied forms. 
Size.—This varies in the genera and in some species. The costals are compara- 
tively large in the genera Platurus, Emydocephalus and Aipysurus, and in Distira 
jervdont, but comparatively small in Hydrus and some Distiva, notably fasciata, cerules- 
