STANTON I 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 n 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 Qth or loth usually being the largest. The 

 other costas 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. tuberculifera 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. veutri- 

 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. Leopold-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. 



2 Zeitschr. Deutsche Geol. Gesellsch., Bd. 43, 1891, p. 418. 



