STANTON I THE MARINE CRETACEOUS INVERTEBRATES. 
21 
largest specimen illustrated is so badly weathered that the anterior set of 
costae has been almost obliterated, and the figure is therefore misleading 
in that respect. The other figures, though smaller, show the sculpture 
better. 
The largest specimen in the collection (PI. IV, Fig. 18) measures 51 
mm. in length, 38 mm. in height and about 19 mm. in convexity of the 
single valves. The corresponding dimensions of another specimen are 
29 mm., 22 mm. and about 9 mm., respectively. 
The only American Cretaceous Trigonia that resembles this one in gen- 
eral appearance is T. hanetiana d’Orbigny, 1 from the Quiriquina beds of 
Chili, which may be easily distinguished by differences in the sculpture, 
the posterior set of costae radiating from the beak instead of from the 
margin of the area, and tending to break up into large irregular tubercles 
toward the ventral margin. T. van Sharpe 2 from the Uitenhage beds of 
South Africa is more nearly related, as its sculpture is of the same type. 
It differs, however, in outline, the posterior end being more prolonged 
and broader, and the anterior ribs are more oblique, while the posterior 
set also have a different inclination. T robinaldina d’Orbigny, 3 a French 
Neocomian species, has somewhat similar sculpture but differs in outline 
and is more convex. 
T. heterosculpta is not easily assigned to any of the described sections 
of Trigonia. The costae are similar to those of the Undulatae but the ill- 
defined area and escutcheon seem to prevent its reference to that group, 
which is said to be characteristic of the Jurassic. 
Locality and position . — From the Ammonite (Belgrano) beds ten miles 
east of Lake Pueyrrydon, represented by about 20 valves. 
Trigonia sp. 
Associated with the preceding from the locality ten miles east of Lake 
Pueyrrydon, are two specimens evidently belonging to a distinct species 
of Trigonia, but too imperfect for specific description, as they do not show 
the character of the posterior end, area and escutcheon. The form is 
rather ventricose and short and the anterior portion of the shell bears nine 
1 Voyage dans l’Amerique Mend., t. Ill, pt. 4, p. 127, pi. 12, figs. 14-16. The species is also 
figured by Steinmann, Neues Jahrb. f. Min., etc., Beilageband X, p. 101, pi. 7, figs. 8, 9. 
2 Trans. Geol. Soc. Lond., 2d Ser., Vol. VII, p. 194, pi. 22, fig. 5. 
3 Paleont. Frang. Terr. Cret., t. Ill, pi. 299, figs. 1, 2. 
