CAMBRIAN BRA CHIOPODA— WALCO TT. 585 



t^oars, cast of apical callosity, and just in front of the latter two slioht 

 tubercles which may be the cast of depressions corresponding to the 

 foraminal pits of Acrofhele. The cast of the brachial valve is also 

 verv instructive in showing- the area, cardinal scars, median ridge, and 

 central scars. 



Observ€(t ions. --Th\i< large tine species differs from its nearest ally, 

 A. ida/ioensis, in the false area and the details of the arrangement of 

 the vascular markings and muscle scars of the interior of the A-alves. 

 The Idaho shells occur in a dark argillaceous shale and were collected 

 by Mr. Spence, of Paris, Idaho. The Eureka, Nevada, specimens are 

 from a shaly limestone. 



Formation and /(>cy///z'^.— Middle Cambrian, near Montpc^lier, 

 Idaho; summit of Prospect Mountain limestone, Prospect Mountain. 

 Eureka District, NcA'ada. 



ACROTRETA GEMMA Billings. 

 Acrotreta gemma Billings, Pal. Foss., I, lS(i5, p. 216, figs. 201a-f. 

 Onghml description.— HheW very small, about 1 line in diameter; one valve nearly 

 flat and the other acutely conical. Dorsal valve very gently convex, nearly circu- 

 lar; sides and front margin uniformly rounded; posterior margin very obtusely 

 angulated 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 halfway to the l)eak; elsewhere the valve is gently convex or 

 nearly flat. 



Ventral valve acutely conical, with a flat triangular area which is peri^endicular 

 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 stride. 



Width of dorsal valve, about 1 line; length, about seven-eighths 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), l)ut that 

 species is twice the size of this and has the area distinctly grooved. 



Oh-e/'vations.— 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 specimens of the pedicle valve 

 were received, but none of the brachial valve, as they could not be 

 found. One specimen in a dove-colored limestone appears to l)olong 

 to a distinct species, which I have named Acrotreta avails. The remain* 

 ing specimens show some variation in the angle of slope of the sides of 

 the pedicle valve, but four of them are similar to the form illustrated 

 by Billings. 



The broad false area with a scarcely perceptible impression of the 

 path of advance of the pseudodeltidium is clearlv shown in one of the 



