NO. 4 CRETACEOUS BRACHIOPODA, ARIZONA — COOPER 3 



Measurements in mm. — Length, 10.6; maximum width, 14.0 ; height, 



34. 



Types.— UoXoiy^e U.S.N.M. No. 124192. 



Horizon and locality. — Mural limestone, from a small hill 300 yards 

 east of U. S. Highway 80, NW^SWiNEi sec. 36, T. 23 S., R. 25 E., 

 Bisbee Quadrangle, Arizona. 



Discussion. — This genus has not hitherto been identified among 

 North American fossils. The laterally elongated muscle scars and short 

 median septum are characteristic. The species is like Craniscus suessi 

 (Bosquet) from the Maastrichtian of Holland in having the apex near 

 the middle and a long, flat posterior slope, but the Dutch species is 

 more swollen anteriorly and is a much deeper shell. 



CYCLOTHYRIS AMERICANA Cooper, new species 

 Plate lA, figures 1-17 



Shell of about medium size for the genus, subtriangular in outline; 

 maximum width at or near the middle ; valves subequal in depth, the 

 brachial valve having a slightly greater depth ; anterior commissure 

 gently uniplicate; surface costellate, costellae numbering about 32 to 

 42 along the anterior margin. 



Pedicle valve moderately convex in lateral profile, with the maxi- 

 mum convexity near the middle ; beak apicate, nearly straight to sub- 

 erect, making an angle of 60° to 80°. Umbo swollen; beak ridges 

 moderately strong and defining a fairly broad, gently concave inter- 

 area ; sulcus originating in the anterior third to half, shallow and oc- 

 cupying about one-third the width. Flanks gently convex, descending 

 steeply to the sides. Deltidial plates conjunct, auriculate; foramen 

 submesothyrid, oval in outline. 



Pedicle valve interior with strong but small teeth; dental plates 

 stout and fairly long, not surrounding the muscle field which is large 

 and broadly elliptical ; diductor scars subreniform in outline ; adductor 

 scars posterior to diductors ; no pedicle collar. 



Brachial valve strongly convex in lateral and anterior profiles ; umbo 

 swollen ; fold low or defined only as a wave of the commissure ; flanks 

 convex ; posterolateral slopes steep. Brachial valve interior with long, 

 slitlike sockets bounded by moderately strong socket ridges ; hinge 

 plate divided; crura curved, short, of radulifer type. 



MEASUREMENTS IN MILLIMETERS 



Length 



Holotype U.S.N.M. No. 124193a 14.0 



Paratype U.S.N.M. No. 124193b 1 1.8 



