STANTON : 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 Quinquina 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. vau 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. heterosctilpta 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 



'Voyage dans 1'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 Sen, Vol. VII, p. 194, pi. 22, fig. 5. 

 3 Paleont. Franc.. Terr. Cret, t. Ill, pi. 299, figs, i, 2. 



