PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 813 
Horizon and locality—Oranda formation in Virginia: In the railroad cut 
4 mile west of Strasburg, Strasburg (15’) Quadrangle. 
Discussion.—This species can be readily recognized by the 3 prominent cos- 
tellae on the surface and the strong geniculation near the middle. The holotype 
briefly noticed by Butts is undoubtedly a youthful specimen which has not yet 
geniculated. The hypotype figured herein shows the full geniculation and indi- 
cates clearly that this species has nothing to do whatsoever with S. rugosa of 
the Eden and Martinsburg formations. 
Sowerbyella ° triradiata Butts is most like Plectambonites quinquecostata 
sublobata Reed from the Balclatchie formation, Girvan District, Scotland. From 
measurements taken from Reed’s figures it is evident that his species is a much 
larger one and apparently less strongly geniculated than the Oranda form. His 
has the same triradiate ornamentation so characteristic of Butts’ species. An 
interior of a brachial valve figured by Reed suggests an interior like that of 
Eoplectodonta rather than Sowerbyella. It shows 3 closely spaced median septa 
and 2 lateral septa separating the adductor scars. No denticulations are noted 
along the hinge in either description or the figures. 
EOPLECTODONTA sp. 1 
Plate 192, E, figures 25-28 
The specimen illustrated is of particular interest because it is probably a young 
one and because it shows clearly the partially denticulate hinge which is regarded 
as an important character of the genus. The small size and gentle contours of 
the profiles of this specimen are also indicators of youth. This specimen was taken 
from the siltstone 30 feet above Echinosphaerites zone in the lower part of the 
Edinburg formation in Virginia, 14 miles south of Wadesville, Winchester (15’) 
Quadrangle. It thus indicates a fairly early appearance of the denticulate hinge 
line. This specimen is of approximately the same age as E. ? dubia, but none 
of the specimens of that species showed clear development of denticles. 
Figured specimen.—117556. 
Subfamily XENAMBONITINAE Cooper, new subfamily 
Sowerbyellidae having a small elevated visceral disk. 
XENAMBONITES Cooper, new genus 
(Greek xenos, strange; ambon, umbo) 
Shell small, semielliptical in outline; hinge wide, cardinal extremities acute ; 
plano- to concavo-convex; anterior commissure deeply sulcate; surface finely 
costellate. 
Pedicle valve with moderately long interarea; anterior with prominent, broad 
median fold ; teeth small, simple ; muscle area wide but short, somewhat rectangu- 
lar in outline; dental plates vestigial; trough corresponding to fold anterior to 
muscle area may or may not be occupied by an inverted V-shaped cavity ; ovarian 
