PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 245 
Measurements in mm.— 
Length Width Thickness 
Holotypen (brachial valve)... sucesso sie 4.0 2.8 0.5? 
Paratype (pedicle valve 109388a) ............ 3.8 2.4 ? 
ou (brachial valve 109388c) ........... 3.4 2.5 ? 
Types.—Holotype: 109388e; paratypes: 109388a,b,f-i, unfigured paratypes: 
109388c,d. 
Horizon and locality.—Sylvan shale (lower 40 feet) in Oklahoma: Middle 
east side sec. 34, T. 1 S., R. 2 E., Murray County, Arbuckle Mountains. Maquo- 
keta chert in Missouri: Decaturville, Camden County. 
Discussion.—This species differs from E. ovalis in its rounder form, some- 
what deeper and more convex valves and the narrower flattened margins, particu- 
larly the posterior margin. On the exterior the minutely radiate umbo is distinc- 
tive. Details of the interior of this species are also very difficult to determine 
because of the thin shell. 
Superfamily AcroTrETACEA Schuchert, 1896 
Family ACROTRETIDAE Schuchert, 1913 
Approximately circular shells with more or less elevated conical pedicle valves 
with small, simple pedicle opening. 
Subfamily AcrotreTINAE Matthew, 1903 
Usually small shells with high conical pedicle valves having a small postapical 
foramen ; brachial valve with marginal beak. 
Identification of the genus Acrotreta is difficult and uncertain because the 
interior details of the genotype are still unknown. The type species is known 
to come from Ordovician rocks, a fact that may exclude many of the Cambrian 
species referred to this genus. It is possible, too, that if the type species of Acro- 
treta comes from the Kunda formation, it may actually have the same structure 
as Conotreta which occurs in rocks of the same age. This point, too, cannot be 
settled until the interior of the type species is known. 
Solution of limestone in acetic acid has opened a new vista in the study of 
these small and difficult shells. The Pratt Ferry limestone, Alabama, has proved 
very prolific of a variety of acrotretids having complicated and unusual struc- 
tures. It is probable that the same methods applied to other bituminous lime- 
stones will produce a still greater variety of forms. This is proving to be true in 
the Cambrian, from which a variety of novel forms has been taken. 
Genus ACROTRETA Kutorga, 1848 
Acrotreta Kutorca, Verhandl. Russ.-Kais. Min. Ges. St. Petersburg, for 1847, pp. 260, 275, 
1848. 
