PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 793 
Kimmswick (15’) Quadrangle; old road 1 mile northwest of Chicago Summer 
School Camp, NW4SW3 sec. 32, T. 37 N., R. 9 E., Weingarten (15’) Quad- 
rangle. 
SOWERBYELLA SILICICA Cooper, new species 
Plate 196, D, figures 20-26 
Sowerbyella sp, Butts, Virginia Geol. Surv. Bull. 52, pt. 2, p. 42, pl. 73, figs. 40, 41, 1942. 
Width of shell at hinge slightly less than twice the length; cardinal extremities 
acute, drawn into small ears; lateral extremities indented just anterior to the 
ears ; anterolateral margin moderately rounded ; anterior margin broadly rounded. 
surface marked by costellae of unequal size ; about 20 to 30 of the larger ones are 
separated by 2 to 4 of the smaller costellae. The median stronger costella of the 
pedicle valve is often the strongest and most prominent of all. 
Pedicle valve gently convex in lateral profile; unevenly convex in anterior 
profile with the median region elevated and the lateral areas or flanks descend- 
ing gently to the lateral margins. Beak low, inconspicuous, protruding only 
slightly posterior to the posterior margin. Umbo narrowly elevated and extend- 
ing anteriorly into the narrow median fold. In the anterior portion of the valve 
and on each side of the fold short sulci often exaggerate the height of the fold. 
Flanks nearly flat or very gently convex in profile. Interarea slightly curved, 
nearly orthocline to gently anacline in position. 
Brachial valve generally moderately to deeply concave, with the greatest con- 
cavity slightly anterior to the middle. Umbo concave; sulcus widening from the 
umbo to the front margin where it is about one-fourth the width at the hinge. 
Width of sulcus dependent on production of feeble to moderately strong plicae 
on each side ; the stronger the lateral plicae the narrower the fold. Flanks bound- 
ing fold nearly flat to moderately concave. Brachial ears faintly concave to flat. 
Interarea strongly hypercline. 
Interior: Pseudodeltidium reduced; median adductor ridge strong, equal in 
length to about one-fourth the length of the valve; ridges bounding anterior 
margin of diductor scars diverging at an angle slightly less than a right angle. 
Brachial valve with lateral septa most elevated at a point slightly anterior to 
the middle; median septum nearly obsolete. Muscle area slightly thickened and 
most elevated at the point where the septa end. 
Measurements in mm.— peed Hinge 
Length length Midwidth width Thickness Height 
TROLOEYV PO isis wiaisge 9 m:a'slars 6.1 5.6 8.8 II.0 i3 2.2 
Paratype (117517b)... 68 6.1 9.5 II.o? 1.4 25 
ss (98213d).... 4.9 4.7 7.0 78 1.0 123 
T'ypes—Holotype: 1175174; figured paratype: 117517b; unfigured paratypes: 
98213a-c, 123457; measured paratype: 982134. 
Horizon and locality—From the upper part of the Arline formation in Vir- 
ginia: At entrance to Porterfield Quarry, 5 miles east of Saltville, Maccrady 
(T.V.A. 218-NW) Quadrangle. 
Discussion—This is a small species characterized by costellae prominently 
grouped into strong ones alternating with groups of finer ones. In this respect 
it is like S. negritus but never attains the large size of that species. It is also 
