ACROTRETIDiE. 683 



This form owes its specific name to the fact that the apex of the ventral valve is strongly 

 incurved. 



Formation and locality. — Passage beds between the Upper Cambrian and the Ordovician: (203a) Lime- 

 stones at base of Pogonip limestone, in the spur on Hamburg Ridge extending out southwest from. Wood Cone, Eureka dis- 

 trict [Hague, 1892, Atlas], Eureka County, Nevada. 



TTpper Cambrian: (12p) About 225 feet (69 m.) above the igneous rocks in the limestones of the Reagan sand- 

 stone, at the northwest extremity of the Arbuckle Mountains, about 4 miles (6.4 km.) east of Homer, Carter County, 

 Oklahoma. 



ACEOTEETA DEFINITA Walcott. 



Plate LXIV, figures 2, 2a-g, 3, 3a-b. 



Acrotreta definita Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, pp. 5S4-585. (Described essentially as below as a 

 new species.) 



For exterior outline, form, and convexity of the valve, the student is referred to the 

 illustrations (PI. LXIV). It is to be noted that the false pedicle groove is very distinct and 

 also broader than that of Acrotreta attenuata Meek. The interior of the ventral valve is beau- 

 tifully shown by casts. One of these (fig. 2c) shows the strong vascular sinuses, cardinal scars, 

 cast of apical callosity, and, just in front of the latter, two slight tubercles, which may be the 

 cast of depressions corresponding to the foraminal pits of Acrothele. The cast of the dorsal 

 valve is also very instructive in showing the area, cardinal scars, median ridge, and central 

 scars. 



Observations. — This large fine species differs from its nearest ally, Acrotreta idaJioensis 

 Walcott, in the false area and the details of the arrangement of the vascular markings and 

 muscle scars of the interior of the valves. The Idaho shells occur in a dark argillaceous shale 

 and were collected by R. S. Spence, of Evanston, Wyoming. The Eureka (Nevada) specimens 

 (PL IjXIV, figs. 3, 3a-b) are from a shaly limestone. 



Acrotreta depressa (PI. LXVI, figs. 8, 8a-c) occurs at about the same stratigraphic horizon, 

 but it has, when uncompressed, a more elevated and sharper apex. 



Fokmation and locality. — Middle Cambrian: (55) Shaly limestone at the top of the Eldorado limestone [Wal- 

 cott, 1908f, p. 184], east slope of Prospect Mountain, in New York Canyon, Eureka district [Hague, 1892, Atlas]; and 

 (57) shaly limestone of the Eldorado limestone [Walcott, 1908f, p. 184], at the 700-foot (213.4 m.) level of the Rich- 

 mond mine, Ruby Hill [Hague, 1892, p. 43, and PL I, opposite p. 116]; both in Eureka County, Nevada. 



(55c and 163) « Spence shale member of the Ute limestone [Walcott, 1908a, p. 8], about SO feet {15.2 m.) above the 

 Brigham quartzite and 2,755 feet (839.7 m.) below the Upper Cambrian, in a ravine running up into Danish Flat from Mill 

 Canyon, about 6 miles {9.6 km.) west-southwest of Liberty, and 15 miles {24-2 km.) west of Montpelier, Bear Lake County, 

 Idaho. 



(4o) Shale about 375 feet (115 m.) above the unconformable base of the Cambrian, in divide at the head of 

 Jackson Creek (locally known as Sheep Creek), a creek flowing into Jackson Lake about 0.5 mile (0.8 km.) south of 

 its northwestern corner, Teton Mountains, Grand Teton quadrangle (U. S. Geol. Survey), Uinta County, Wyoming. 



ACEOTEETA DEPEESSA (Walcott). 



■". Plate LXVI, figures 8, 8a-c. 



Acrotreta gemma depressa Walcott, 1889, Proc. U. S. Nat. Mus. for 1888, vol. 11, pp. 441-442. (Reason given for 

 separation as a new variety.) 



Linnarssonia sagittalis Walcott, 1889, idem, p. 442. (Mentioned.) 



Acrotreta gemma depressa Walcott, Matthew, 1902, Trans. Roy. Soc. Canada for 1902, 2d ser., vol. 8, sec. 4, No. 3, 

 p. 109. (Copies Walcott, 1889c, p. 441, and proposes the species A. depressa.) 



Acrotreta depressa Walcott, 1908, Canadian Alpine Joiu-nal, vol. 1, No. 2, p. 245, PL I, figs. 3, 3a-c. (No text refer- 

 ence. Figs. 3, 3a-c are copied in this monograph, PL LXVI, figs. 8c, 8c', 8a, and 8, respectively.) 



A group of depressed ventral valves in an arenaceous argillaceous shale led me [1889c, 

 p. 441] to distinguish the shell as a variety of Acrotreta gemma Billings. Material received 

 since 1889, from a black and finer shale, shows an elevated ventral valve much like that 

 of A. idaJioensis alta Walcott, but at present I am unable to decide that the typical form of 



a 163 is the type locality. 



