A MARINE EARLY CRETACEOUS FAUNA. 
15 
Remarks: Only one specimen, the holotype, has been collected. It 
is a relatively small form for the genus and thereby possibly is immature. 
In comparing it with other species, particularly the younger stages 
of other species, attention is drawn to the thinness of the shell in this 
form ; to its sharply triangular outline and very sharp marginal carina 
in its early stage; to the very acute Y-ribbing and the straightness of 
the Y-axis. In such features it is in sharp contrast with such forms as 
Z. crassitesta van Hoepen and Z. inconstans van Hoepen with thick tests 
and less incisive details of form. It is to Z. v-scripta that it shows most 
resemblances. Some smaller specimens of that species figured by Kitchin 
(1903, pi. YIII, figs. 1, 2) are very comparable, being similarly 
compressed and having also a more strictly scalene outline and a sharp 
marginal carina, while the ornament of the cardinal area also generally 
agrees. The two, although close, are not con-specific ; for in T. v-scripta 
the Y-axis is slightly curved and there is a median carina also on the 
area. The development of the costae in the early stages of these two 
species is rather similar. T. haughtoni, although thicker and more 
inflated, also may be compared for its relatively incisive details in the 
early stages and in the development of the early costae. But there the 
V-ribs although more acute than the genotype are developed along 
an axis with a very marked curvature. 
The holotype of Z. limatula has no suggestion of gerontic characters. 
Indeed in the preciseness of shape, details of ribbing and the sharpness 
of carina, it has a particularly youthful appearance. Whether larger 
specimens will show a cessation of the Y-ribs in the very last stage 
remains to be proved. It is not to be concluded, of course, that 
apparently youthful characters indicate an early species of the genus. 
Spath (1935, p. 185) rightly has pointed out that arguments about 
age based purely on the apparent stage of evolutionary development 
within a group of Trigonias are apt to be misleading and in most 
instances are valueless. 
Genus Pisotrigonia van Hoepen, 1929B. 
(Genotype by original designation: P. salebrdsa van Hoepen p. 20, pi. Y, 
figs. 1-6 from the Ndabana beds of Zululand.) 
(Synonym: Rinetrigonia van Hoepen 1929B, genotype Trigonia 
ventricosa Krauss. Salebrosa and ventricosa probably are identical, 
in which case ventricosa has precedence.) 
Kitchin, who did such admirable work on the Trigonias of the 
Oomia beds in Kachh and of the Uitenhage beds of South Africa, was 
not able to separate forms from these two regions that he referred to 
T. ventricosa Krauss. Yan Hoepen not only has made them specifically 
distinct but placed them in different genera. Rennie (1936, p. 330) 
has protested against this as wrong and unnatural, and I agree with 
him in recognising one genus only for the group and probably a common 
species in these two regions. Of the two names Pisotrigonia' has preced- 
ence, since Rinetrigonia was proposed merely in the last sentence of 
the description of Pisotrigonia. 
Pisotrigonia occurs in the same beds as lotrigonia — that is, in the 
Uitenhage beds of South Africa, the Ndabana beds of Zululand and the 
Oomia beds of Kachh; so that the same reasoning that was used above 
in determining the age of lotrigonia applies to Pisotrigonia. 
D 
