STANTON : THE MARINE CRETACEOUS INVERTEBRATES. 
19 
the margin ; remainder of the shell bearing about 22 to 24 strong costae, 
radiating from the margin of the area and divided into two rather distinct 
sets. The anterior 10 or 11 are very strong, distant, coarsely and irregu- 
larly tuberculate, curved forward near the margin of the shell, occupying 
the inflated anterior two thirds. Successive costae become larger and 
more nearly straight, the 9th or 10th usually being the largest. The 
other costae on the contracted posterior third of the shell are much finer, 
more closely arranged and nearly straight or slightly irregular and sinuous, 
without tubercles, and directed obliquely downward and backward. The 
surface also bears rather conspicuous, closely arranged lines of growth. 
The figured type, which has lost a small portion of the posterior end, 
measures 73 mm. in length, 63 mm. in height, and about 57 mm. in con- 
vexity of the two valves. The corresponding dimensions of another 
specimen are 82 mm., 66 mm., and 60 mm., respectively. In each case 
the length is measured from the front margin to the posterior end, and the 
height somewhat obliquely from the beak to the most prominent part of the 
ventral margin. 
This species belongs to the section Scabrae, which is characteristic of 
the Cretaceous, and it is somewhat closely related to T. aliformis Park- 
inson. The form which most closely resembles it, however, is T. ventri- 
cosa (Krauss), 1 from the Uitenhage beds of South Africa. 
Comparisons have been made with some small specimens collected by 
Dr. Holub on Zwartkop river, as well as with the published figures, and 
while the general resemblance is very great, the Patagonian form differs in 
being somewhat longer and less inflated, and the tubercles on the anterior 
ribs are coarser, less regular and more distant. T. tuber culif era Stoliczka 
from the Upper Cretaceous Trichinopoly beds of southern India is also 
similar in general form and sculpture, but it is still shorter than T. ventri- 
cosa and the costae on the posterior portion are coarser and not so nu- 
merous. 
In beds referred to the Upper Neocomian at Arroyo Pequenco, Argen- 
tine Republic, several hundred miles north of these Patagonian localities, 
Dr. Bodenbender collected a Trigonia, listed by Behrendsen 2 as Tri- 
1 Nova Acta Acad. Caes. Leopolcl-Carolin. Nat. Cur., Vol. 22, p. 456, pi. 49, figs 2a-2d. Bet- 
ter figures have been published by Lycett in Brit. Foss. Trigoniae, p. 119, and by Stoliczka in 
Cret. Fauna of S. India, vol. 3, pi. 15, figs. 9, 9 a. 
2 Zeitschr. Deutsche Geol. Gesellsch., Bd. 43, 1891, p. 418. 
