ACROTRETID^. 685 



The specific name is given in memory of Dr. Ebenezer Emmons, who studied and wrote 

 of the Bald Mountain section. 



Formation and locality. — Lower Cambrian : (35a) Shaly limestone on the west slope of the summit of Bald Moun- 

 tain, S miles {4.S km.) north-northwest of Greenwich, Schuylerville quadrangle ( U. S. Geol. Survey); and (38a) limestone 

 2 miles (3.2 km.) south of North Granville, on the road which turns south from the road running between that village 

 and Truthville, 4 miles (6.4 km.) west-northwest of Granville, Fort Ann quadrangle (U. S. Geol. Survey); both in 

 Washington County, New York. 



ACROTEETA GEMMA BillingS. 



Plate LXVI, figures 1, la-b. 



Acrotreta gemma Billings, 1865, Geol. Survey Canada, Paleozoic Fossils, vol. 1, pp. 216-217, figs. 201a-f. (Described 



as below as a new species. The specimens represented by figs. 201b, d, e, and f are redrawn in this monograph, 



PI. LXVI, figs. 1, la-b, respectively.) 

 Not Acrotreta gemma Walcott [1884b, p. 17; 1886b, p. 98; 1891a, p. 608; or 1899, p. 449]. (The specimens referred to 



Acrotreta gemma Billings in the references mentioned have been referred in this monograph to Acrotreta attenuata, 



A. curvata, A. idahoensis aha, A. primxva, and A. pyxidicula, which see.) 



The original description by Billings follows: 



Shell very small, about 1 line in diameter; one valve nearly flat and the other acutely conical. Dorsal valve 

 very gently convex, nearly circular; sides and front margin uniformly rounded ; posterior margin very obtusely angu- 

 lated at the beak, on each side of which a portion of the cardinal edge, equal to one-fourth of the whole width of the 

 shell, is nearly straight; umbo very small; beak apparently depressed to the hinge line and not projecting beyond it; 

 cardinal angles compressed, broadly rounded; a wide, shallow, mesial sinus extends from the front margin about half- 

 way to the beak ; elsewhere the valve is gently convex or nearly flat. 



Ventral valve acutely conical, with a flat triangular area which is perpendicular to the plane of the lateral margin, 

 its base half the width of the whole shell. In the apex of this valve there is a minute circular aperture, and in one 

 specimen a dark line extends from it down the middle of the area, which appears to represent the foraminal groove 

 of this genus; but in two other specimens of the ventral valve, with the area well preserved, there is no indication of 

 a groove. Surface with very fine concentric striae. 



Width of dorsal valve about 1 line; length about eight-ninths of a line. The height of the ventral valve is about 

 1 line. 



The form of this species is very like that of A. subconica Kutorga, but that species is twice the size of this and has 

 the area distinctly grooved. 



Observations. — By the courtesy of Prof. J. F. Whiteaves, of the Geological Survey of 

 Canada, I have had the opportunity of studying the type material of this species. Nine speci- 

 mens of the ventral valve were received, but none of the dorsal valve, as they could not be 

 found. One specimen in a dove-colored limestone appears to belong to a distinct species, 

 which I have named Acrotreta ovalis. The remaining specimens show some variation in the 

 angle of slope of the sides of the ventral valve, but four of them are similar to the form illus- 

 trated by Billings [1865a, p. 216]. 



The broad false area, with a scarcely perceptible impression of the path of advance of 

 the pseudodeltidium, is clearly shown in one of the specimens, and faintly in others. Two 

 shells have the apex broken off so as to expose the cast of a minute apical callosity and a small 

 portion of the side of the cast of the main vascular sinus (PL LXVI, fig. lb). 



Acrotreta gemma belongs to the A. subconica Kutorga group of the genus, having a high 

 ventral valve and a distinct and broad false area. It differs from A. subconica in having a less 

 elevated ventral valve and in the apex being in front of the posterior margin instead of extending 

 over it. Acrotreta gemma has no strong specific relations with the forms from the Rocky 

 Mountains that I identified with it. A. attenuata Meek has a high ventral valve, but the false 

 area is practically absent, a narrow, deep sulcus taking its place. A. idahoensis alta Walcott 

 has the elevation and false area, but differs in the details of the area and the outline of the cross 

 section of the ventral valve. It was this form that led me [1884b, p. 17] to consider th&t A. gemma 

 occurred in Nevada, and with the slight knowledge that I then had of the genus and species a 

 -wide variation of form was given to A. gemma. As far as now known to me, the species is 

 restricted to the type locality. 



Formation and locality. — lower Ordovician: (314d [Billings, 1865a, p. 217]) Limestone of Division P of the 

 "Quebec group," 4 miles (6.4 km.) northeast of Portland Creek, Newfoundland. 



