202—48 INDIAN TERTIARY AND POST-TERTIARY VERTEBRATA. 
that of the existing forms> as well as from E. sivalensis, by the more acutely pointed 
and more widely separated tubercles, which are always j^erfectly conical, and have 
no tendency to the irregular sha])e which they present on the hinder part of the 
nuchal scute of the other species. As the specimen agrees in this respect with the 
marginal scute described above, it most probably belongs to the same species. 
Distinctness and affinities. — From the evidence of the two specimens just described , 
there seems no reasonable doubt as to the existence of a fourth well-defined species 
of Siwalik Emi/da., to which it is j)roposed to apply the name of E. palccmdica. 
The Indian Museum ^^ossesses specimens of the laler marginal scutes of various 
Siwalik forms of Emyda, but it is not easy to refer them to tlieir respective species. 
It may be hoped that future researches • in the Punjab will bring to light more 
perfect sj)ecmiens of this and the two preceding species which will more fully 
illustrate their affinity. Fragmentary as they are, the remains on which these 
species are founded are very imjDortant, as serving to indicate the great development 
and diversity of structure which this peculiar Oriental genus attained in pliocene 
times. 
Distribution. — All the remains referred to this species have been obtained from 
the Punjab, within the catchment basin of the Indus. 
Genus II. TRIONYX, Geoffrey.^ 
Definition. — Shell very similar to that of Emyda.^ but with a coarser sculpture, 
and without marginal scutes : the callosities on the plastron may be eitlier four or 
five in number, the two anterior (epiplastral) callosities of Emyda being invariably, 
and the azygos (entoplastral) one sometimes absent. 
The genus and its allies presents extreme difficulty to the palaeontologist (and 
more especially in the case of Indian forms) owing to the close similarity existing in 
the structure of the shell of most of the species, and the circumstance that a large 
series of macerated shells of the existing species is not generally attainable. The 
structure of the cranium of the existing Ghitra indiccd is so different from that of the 
typical Trionyx gangeticus^ that it seems almost imperative to follow Gray in regarding 
the' former as generically distinct : the shell of Ghitra apparently, however, differs 
from that of the Indian species of Trionyx solely by its superior size and the absence 
of an anterior azygos (entoplastral) callosity.'* Delochelys cantori has likewise such a 
peculiar cranium that there seems good reason for separating it generically : its 
shell according to Mr. Theobald® has no azygos callosity, but one of the specimens 
in the British Museum shows a very small one. All the other existing forms may be 
referred to Trionyx. 
Distribution. — At the present day Trionyx has a very wide distribution ; and 
numerous fossil forms have been referred to it from various tertiary formations 
1 ‘Ann. d. Museum,’ vol. XIV. p. 1 (1809). 2 Gray, “ Catalogue of Shield-Ueptiles,” pt. 1. pi. XLI. 
2 Ihid, pi. XLII. fig. 1. . ^ Vide Theofiald, “ Catalogue of the Eeptiles of British India,” p. 26. 
i> Op. cit., p. 27. 
