PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 469 
shallow and producing a small tongue. Flanks slightly convex with fairly steep 
posterolateral slopes and moderately steep anterior and lateral slopes. Interarea 
short, curved, apsacline. Delthyrium narrow. 
Brachial valve strongly convex in lateral profile with the maximum convexity 
at about the middle. Anterior profile also strongly convex, this valve having 
greater depth than the pedicle one. Fold visible only as a wave of the anterior 
margin in the brachial direction. Median region inflated, with steep slopes to all 
the margins. Umbonal region somewhat inflated. Interarea short and slightly 
apsacline. 
Interior of the pedicle valve with a broadly triangular muscle field having a 
wide adductor track. 
Measurements in mm.—Holotype, length 18.6, brachial length 18.2, width 27.0, 
hinge width 19.0, thickness 16.6. 
Types.—Holotype: 48755a; figured paratype: 110652a. 
Horizon and locality—Pierce and Ridley formations in Tennessee: At 
Murfreesboro. 
Discussion.—This species attains a large size and is characterized by its inflated 
valves, gentle contours, and fine costellae. It is a rare species, at present known 
from the Pierce and Ridley (?) limestones of the Central Basin of Tennessee. 
In size this species suggests M. globosa, but it is actually smaller than the larg- 
est of that species. Furthermore, the exterior contours and profiles are quite 
different, the East Tennessee species being much less inflated. 
MIMELLA BOREALIS (Billings) 
Plate 85, E, figures 38, 39 
Orthis borealis Bittincs, Canadian Nat. Geol., vol. 4, p. 436, fig. 14, 1859.—LeEsLEy, Geol. 
Surv. Pennsylvania, Rep. P 4, p. 510, figs., 1880. 
Hebertella borealis (Billings) RaymMonp, Ann. Carnegie Mus., vol. 7, p. 241, figs. 13, 14, IQII. 
Types.—Holotype: G.S.C. 1035¢. 
Horizon and locality—St. Martin formation in Quebec Province, Canada: 
Caughnawaga, La Prairie County. 
MIMELLA COSTELLATA Cooper, new species 
Plate 91, A, figures 1-7a 
Shell small for the genus, subelliptical to subrectangular in outline; sides 
rounded, anterior margin subtruncate. Hinge narrower than the greatest shell 
width which is at about the middle. Anterior commissure broadly uniplicate ; 
lateral commissure deflected toward the brachial valve near the cardinal extremi- 
ties. Surface evenly and finely costellate, 20 costellae in a space of 5 mm. at the 
front margin. 
Pedicle valve strongly convex in posterior half but flattened anteriorly when 
seen in lateral profile. Anterior profile moderately convex with the greatest con- 
vexity in the median portion. Umbonal region strongly convex. Sulcus shallow 
and narrow, not conspicuous and originating near the middle. Flanks flattened 
