584 PROCEEDINGS OF THE NATIONAL MUSEUM vol. xxv. 



ACROTRETA CONULA, new species. 



Shell small. Pedicle valre excentrieally cone-shaped, with a broadly 

 ovate transverse aperture. The apex is situated at the posterior 

 fourth of the distance between the front margin and the posterior 

 edge of the slightly indicated false area. A minute pedicle aperture 

 occurs at the summit of the apex. Brachial valve convex with the 

 minute beak at the posterior margin. The cast shows a well-detined 

 median ridge with the central nuiscle scars well back on the valve. 



Surface marked by tine concentric stria? and lines of growth. 



Foriaati(>n andlocal'dy.- — Upper Cambrian, Olenus truncatii.-t zone, 

 Oland, Sweden. 



ACROTRETA CONVEXA, new species. 



Shell small, subcircular, valves convex. Pedicle valve most elevated 

 at the low apex which is above the slightly transverse posterior mar- 

 gin. Brachial valve somewhat less convex than the pedicle valve, 

 with its greatest elevation at the posterior third beak, minute, mar- 

 ginal. Length and width of shell 1 mm. Shell substance very thin 

 but not proportionally thinner than other larger forms. Surface 

 marked by fine concentric stride and lines of growth. 



Casts of the interior of the pedicle valve show a minute apical cal- 

 losity and cardinal scars. The cast of the brachial valve has a long 

 median furrow and minute cardinal scars. 



This minute shell occurs in a fine-grained sandstone interbedded in 

 the shales of the Upper Cambrian. It is characterized mainly by the 

 convexitj' of the brachial valve and the low pedicle valve. 



Formation, and locality. — Upper Camljrian, Gillis Hill, Salmon 

 River, 13 miles south of Marion Bridge, Cape Breton, Nova Scotia. 

 Collected by Mr. S. Ward Loper. 



ACROTRETA CURVATA, new species. 



This is one of the so clearly defined species that it is possible to illus- 

 trate all that is known of it with the exception of the ver}' fine con- 

 centric surface striiB. It belongs with A. idalwensis and other species 

 with a broad false area. It dift'ers from described species in the 

 incurved apex of the pedicle valve and the general aspect of the two 

 valves. 



Fofuiation and locality. — Upper Cambrian, passage beds l)etween 

 Cambrian and Ordovician near base of Pogonip limestone, Ham1)urgh 

 Ridge, Eureka District, Nevada. 



ACROTRETA DEFINITA, new species. 



The false pedicle groove is very distinct and also broader than that of 

 A. attenuata. The interior of the pedicle valve is beautifully shown 

 by casts. One of these shows the strong vascular sinuses, cardinal 



