PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 879 
MICROTRYPA ALTILIS Wilson 
Microtrypa altilis Witson, Trans. Roy. Soc. Canada, ser. 3, vol. 39, sec. 4, p. 144, pl. 2, 
figs. 10-12, 1945; Geol. Surv. Canada, Bull. 8, p. 111, pl. 10, figs. 10-12, 1946. 
Type.—Holotype G.S.C. 7649. 
Horizon and locality—lLeray-Chaumont formation, Stewart Quarry, Rock- 
land, Ontario. 
MICROTRYPA ? NASUTA Wilson 
Microtrypa ? nasuta Witson, Trans. Roy. Soc. Canada, ser. 3, vol. 30, sec. 4, p. 146, pl. 2, 
figs. 13, 14, 1945; Geol. Surv. Canada, Bull. 8, p. 112, pl. 10, figs. 13, 14, 1946. 
Types.—Holotype: G.S.C. 7641; paratype: G.S.C. 7641Aa. 
Horizon and locahty—Rockland formation, Angegardien road, 4 miles west 
of L’ Orignal, Ontario. 
Genus HOLTEDAHLINA Foerste, 1924 
Plate 2209, B, figures 6-8 
Figures of the interior and exterior of this genus are introduced for compari- 
son with Furcitella which has been confused with Holtedahlina. 
Types.—Figured specimens: 117744, 117747a,b. 
PLATYMENA Cooper, new genus 
(Greek platys, flat; mene, crescent) 
Shells attaining a width of about # inch; subrectangular in outline ; planocon- 
vex to slightly concavo-convex ; hinge as wide or wider than the midwidth. Sur- 
face multicostellate, costellae unequal in size, a larger one setting off one to 
several finer ones. Pseudopunctae finely matted. 
Pedicle valve with stout teeth, no dental plates, but a thickened rim extends 
anteriorly from the dental ridge and nearly encloses the small muscle field; di- 
ductor scars subflabellate ; adductor scars small, associated with a thickened ridge 
on the inner sides of the diductors ; median ridge, low, short, not extending an- 
terior to the anterior ends of the diductors. Valve thickened inside the margin. 
Pseudodeltidium small; chilidium small. 
Brachial interior with delicate bilobed cardinal process; notothyrial platform 
moderately thick, prominent ; brachiophores long, slender blades enclosed by deli- 
cate cups to form broad sockets; median ridge short; adductors separated by 
2 oblique, low ridges on each side of the median ridge. Visceral region sur- 
rounded by a thickened margin. 
Genotype.—Platymena plana Cooper, new species. 
Discussion.—Platymena is characterized by its planoconvex profile, the small 
muscle area enclosed by strong ridges, a minute bilobed cardinal process, and 
obscure median ridge. It differs from Macrocoelia and Opikina in the profiles 
of the valves and the delicacy of the cardinalia. It also differs in the possession 
of a broad and thickened border around the visceral region of both valves. 
