PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 287 
Discussion—This species is characterized by its circular form, nearly sym- 
metrical conical profiles, and very fine spines. 
Genus PETROCRANIA Raymond, 1911 
Petrocrania RaymMonp, Ann. Carnegie Mus., vol. 7, No. 2, p. 229, I9QII. 
This genus is of rare occurrence in early Middle Ordovician rocks except lo- 
cally, where it may be common. As far as known no member of this genus has 
yet been taken from high Pogonip or early Chazyan rocks. The oldest species, 
therefore, is Petrocrania prona (Raymond) from the Crown Point limestone of 
New York. 
A point of considerable interest in connection with species of Petrocrania 
found in the rocks under consideration is the fact that at least as far as the Na- 
tional Museum collections are concerned no pedicle valves have yet appeared. In 
discussing P. ulrichi, Winchell and Schuchert (1895, p. 375) remark on this fact 
but suggest that the pedicle vaive was convex. This suggestion was supported 
by the statement that the two valves figured by Hall and Clarke appeared to 
have different musculature and indicated pedicle and brachial valves. The muscu- 
lature as restored for the pedicle valve is not usual for the genus, and it is prob- 
able that the restoration and identification of the valve are in error. 
PETROCRANIA CICATRICULA Willard 
Petrocrania cicatricula Wittarp, Bull. Mus. Comp. Zool., Harvard Coll., vol. 68, No. 6, 
p. 260, pl. 3, fig. 4, 1928. 
Type.—Holotype: M.C.Z. 8590. 
Horizon and locality —“Ottosee” (probably Hogskin) : Quarry north of Lut- 
trell, Tenn. 
PETROCRANIA DIXONENSIS Cooper, new species 
Plate 27, E, figures 10, I1 
Under this heading is placed a single species of fairly large size from the 
Platteville limestone. It has the following characters: Elliptical in outline with 
the width about 14 times the length, posterior margin short and flat; lateral and 
anterior margins broadly rounded; beak almost marginal ; posterior slope steep, 
and slightly concave; anterior slope long, strongly convex; anterior profile 
broadly convex. Distance of beak from posterior margin 1.5 mm. 
Measurements in mm.—Holotype, length 13.6, width 16.5, thickness 3. 
Type.—Holotype: 45215. 
Horizon and locality—Platteville formation in Illinois: At Dixon, Dixon 
(15’) Quadrangle. 
Discussion—This species differs from P. trentonensis Hall in its large size, 
the more marginal apex, and the strongly inflated and proportionately longer 
anterior slope. 
