PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 481 
Median region strongly swollen and with steep but short slopes to the lateral 
margins and cardinal extremities. 
Interior of pedicle valve with well-developed dental plates and strongly im- 
pressed muscle field. Diductor scars long and narrow; adductor ridge promi- 
nent, elevated, and somewhat elongated; adductor scars crescentic. Brachial 
valve with short, elevated median ridge; notothyrial cavity deep; brachiophores 
short, supporting plates thick, moderately elevated anteriorly ; cardinal process 
a simple ridge. 
Measurements in mm.— 
Brachial Width Hinge 
Length length width Thickness 
TOLOEY DE te sist sie ceteris: 11.6 10.4 14.2 10.6 8.9 
Paratype (110804a) ......... 11.6-+ 10.7 13:3 10.8 8.2 
Types.—Holotype: 110786; figured paratypes: 110804a, 110797a,b; unfigured 
paratypes: 110804b-e, 110797c. 
Horizon and locality—Crown Point formation in New York: Sloop Bay, 
Valcour Island, Lake Champlain, Plattsburg (15’) Quadrangle. 
Discussion.—This species is characterized by its narrow hinge, strongly convex 
valves and numerously intercalated costellae. It differs from M. vulgaris Ray- 
mond in its convex valves, numerous costellae, narrow hinge, and less strongly 
apsacline interarea. It suggests M. nuclea but is more convex, has a less 
apsacline interarea, somewhat narrower hinge, and more numerous costellae. 
MIMELLA PIGER (Billings) 
Plate 99, D, figures 20-23 
Orthis piger Brttincs (in part), Canadian Nat., vol. 4, pp. 442, 443, 1859. 
Plectorthis ? piger (Billings) TwENHOFEL and Wuittnc, Geol. Soc. Amer. Special Pap. 11, 
p. 48, pl. 7, figs. 7-9, 1938. 
Cooper examined the specimen figured by Twenhofel and Whiting and identi- 
fied it as a Mimella rather than a Plectorthis. The ribbing, profile, and length 
of interarea of the pedicle valve are wrong for Plectorthis. It does not seem to 
be the same as “Plectorthis” exfoliata (Raymond). 
Type—Hypotype: Y.P.M. 15823. 
Horizon and locality—Mingan formation in Mingan group: On Parroquet 
and Eskimo Islands. 
MIMELLA SIMILIS Cooper, new species 
Plate o1, B, figures 8-12; plate 91, C, figures 13-18; plate 92, D, figures 23-28 
Shell of about medium size for the genus, wider than long; subrectangular in 
outline; sides moderately rounded; anterior margin broadly rounded; anterior 
commissure nearly rectimarginate to gently uniplicate. Hinge narrower than the 
greatest shell width which is anterior to the middle; cardinal extremities form- 
ing a small obtuse angle. Multicostellate, costellae narrowly rounded and ele- 
vated, closely crowded, 3 in the space of 1 mm. at the front margin. 
