PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 593 
wider than the other striae and bearing a faint costella. Cruralium shallow and 
wide; fulcral plates well developed; brachiophores short, crescentic in section ; 
median septum long, elevated. Interarea moderately well developed. 
Measurements in mm.—Holotype, length 11.7, brachial length 10.8, width 11.2, 
hinge width 4.9, thickness 8.3. 
Types—Holotype: 117174g; figured paratypes: 117174d-f,h, 117175b-d,h; 
unfigured paratypes: 117174a-c,1-1, 117175a,e,f. 
Horizon and locality.—Pogonip group (Orthidiella zone) from the mottled 
zone 700 feet below the Eureka quartzite-in Nevada: At the base of the first 
ridge east of Frenchman Flat, Las Vegas (1°) Quadrangle. 
Discussion.—No other species like this one is now known. 
PERIMECOCOELIA Cooper, new genus 
(Greek peri, around; meco, long; koilos, hollow) 
Transversely and broadly elliptical in outline; biconvex but with the brachial 
valve having the greater depth; strongly uniplicate; paucicostate, the posterior 
half smooth and the anterior half or less costate. 
Pedical valve with an elongate spondylium, narrowed anteriorly, and supported 
by a short median septum. 
Brachial valve with long cruralium supported by a short median septum not 
continued anteriorly ; brachiophores short and stout ; fulcral plates small, defining 
a small socket. Adductor field longitudinally elliptical, partially divided by the 
cruralium. 
Genotype.—Perimecocoelia semicostata Cooper, new species. 
Discussion—This genus externally is very suggestive of Parastrophina be- 
cause of its transversely elliptical outline, strongly swollen brachial valve, and 
paucicostate ornamentation. The transverse outline separates the genus from 
most of the camerellids which are usually longitudinally oval or somewhat 
triangular. 
Inside the pedicle valve the spondylium is like that of Camerella and its allies 
in its elongate form and the narrower anterior. Many specimens of Perimeco- 
coelia have the spondylium nearly divided into two chambers by processes grow- 
ing inwardly from near the center of the structure. No specimens have been 
found in which the processes united, although in a few instances they have almost 
done so. 
The interior of the brachial valve is most suggestive of that of Parastrophina 
with its long cruralium. However, it differs from that genus in not having the 
alate plates and in possessing a much shorter median septum. The cruralium of 
Perimecocoelia is quite unlike that of Camerella which is short and confined to 
the beak and supported by a long median septum. 
In all the many specimens of the brachial valve available for study the brachio- 
phores are short and stubby. Furthermore, they appear to lie under the fulcral 
plate which forms an S-bend from the inner wall of the valve to the posterior 
