A MARINE EARLY CRETACEOUS FAUNA. 
17 
Genus Indotrigonia Dietrich 1933. 
(Genotype, by original designation: Trigonia smeei J. de C. Sowerby 
from the Oomia beds of Kachh.) 
Indotrigonia (?) sp. Pl. I, figs. 4a, b; 6. 
One external mould with the ornament abraded (pl. 1, fig. 4) is 
of a shell with straight ventral and dorsal margin and uniformly 
rounded anterior margin, with the umbones well towards the anterior 
end. The ornament is of strong concentric ribbing. There is a wide, 
shallow postero-ventral sulcus, a broad carina and a shallow trough-like 
posterior cardinal area. In and posterior to the sulcus the ribbing 
apparently ceases and the surface is marked only with growth striae. 
One other external mould (fig. 6) apparently of the same species shows 
the strength and simplicity of the concentric anterior ribbing. 
This apparently is one of the purely concentrically ornamented 
Trigonia groups that, as suggested above in the section on Iotrigonia , 
are probably end-points of several lineages. Four such genera have been 
erected so far for Cretaceous forms — Butitrigonia and Pleurotrigonia 
van Hoepen, Indotrigonia Dietrich and Sphenotrigonia Rennie. 
Although it is not definite that it belongs to Indotrigonia , the species 
is perhaps more comparable to members of that genus than any other. 
Forms belonging to it have very strong concentric costae, although 
there may be rudiments of other ornament in the umbonal regions (the 
umbo of this specimen is too abraded to register the early ornament). 
From the two species known in the genus (both from the Oomia beds of 
Kachh, with the genotype occurring also in Tanganyika) this form is 
distinct. The genotype, T. smeei (see particularly Kitchin, 1903, pis. Ill, 
fig. 9;. IV, figs. 1-3) has a very narrow sulcus and a wide triangular 
area. T, crassa Kitchin (1903, p. 44, pis. IV, figs. 4-6; V, figs. 1-3) is 
