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 Aipysurus 

 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 allied forms. In order 

 to obtain the best results, I count these scales in three situations, viz., anteriorly, i.e., 

 two headslengths behind the head, in mid body, and posteriorly, i.e., two headslengths 

 before the anus. The terms " anterior " and " posterior " used throughout this mono- 

 graph are therefore precise. In Distira 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), Aipysurus 

 (except Icevis) (17 to 19), and Distira jerdoni (19) ; most numerous in Thalassophis 

 annandalei (goto 100), Enhydrina (50 to 70), Hydrus (45 to 62), and Astrotia (48 to 

 59). In Distira 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 Aipysurus, 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, Distira 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 Distira. In the genera Platurus, Emydocephalus, Aipysurus 

 and Astrotia imbrication is pronounced in the whole body length, also in Distira jerdoni. 

 In Hydrus and Enhydris the costals are juxtaposed throughout, and in certain species, 

 notably Distira 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. omata, D. ccerulescens , 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 

 jerdoni, but comparatively small in Hydrus and some Distira, notably fasciata, ccerules- 



