NO. 1395. CAMBRIAN BRACHIOPODA—WALCOTT. 265 



sul valve, more strongly incurved umbo and apex of the ventral valve, 

 and the general appearance of convexity and rotundity of the ventral 

 valve. When the specimens of the two species are imperfect, or the 

 yoiuig shells of /•*. luicJutaensis occur in association with the shells of 

 P. indianola it is difficult to determine positively to which species the}' 

 belong. 



Formation and locality. — Upper Cam})rian. Top of Reagan forma- 

 tion, west side of Honey Creek, near SE. corner sec. ;^5, T. 1 S., 

 R. IE., Ardmore quadrangle, , northwest extremity of Arbucklc 

 Mountains, al)Out 4 miles east of Homer. Indian Territory, Reagan 

 gveensand, two-thirds of a mile east of Canyon Creek, 15 miles north- 

 west of Fort Sill, SE. i NE. i sec. 2, T. 4 N., R. 13 AV., Wichita 

 Mountains, Oklahoma Territory. 



Middle Cambrian. Reagan Formation, SW. i sec. 17, T. 4 N., R. 

 12 W., Wichita Mountains, 11 miles northwest of Fort Sill, Okla- 

 homa Territor3^ Potosi limestone, near Potosi, Missouri. Limestone 

 on Cold Creek, north end of gorge, opposite north end of Sponge 

 Mountain, Llano County, Texas. 



PLECTORTHIS JOHANNENSIS Matthew. 



Orthmna (?) johannenm Matthew, 1891, Trans. Roy. Soc. Can., IX, 1891, p. 

 49, pi. XXII, figs. i;^a-c. 



Origwal description. — Shell subquadrate, doubly convex, rather flat, very thin. 



Dorsal valve depressed at the sides and front, and having a broad, shallow median 

 sinus. The valve is about one-quarter wider than long, and the hinge line is nearly 

 as long as the length of the shell. Umbo slightly elevated, hinge plate weak and thin. 



Ventral valve depressed at the sides, and having a few median ridges running from 

 the umbo to the front of the shell. This valve is somewhat geniculated at two-fifths 

 of its length from the umbo, and from the bend faintly raised ridges diverge to the 

 anterior angles of the shell; behind the geniculation the surface of the shell is marked 

 l»y faint undulations similar to those of Strophb)i)ena rJinmboidalis. 



The umbo is not prominent, but the back of the valve is regularly curved and the 

 area rather low. 



Sculpture. — Closely set strife radiating fr(nii tiie umbo and faint concentric stripe 

 mark the shell. 



Size. — Length of the ventral valve, 9 mm. ; width, 10 mm. Length of dorsal, 8 

 mm. ; width, 10 mm. 



Horizon and locality. — [Upper Cambrian C. D. W.] Limestone lentilles in the 

 black shales of division .'^a, Germaine street, St. John. 



This species has a low umbo for an fh'thisina, and in its form recalls the genus 

 Slrophoiuena, as also do the concentric undulations that are found on the back of the 

 ventral valve. The dorsal valve, however, is convex, and the area of this and the 

 ventral valve too high for a Strophomena. It does not appear to agree with any 

 described species of Orthisina. It resembles 0. orientalis White, somewhat in form, 

 t)ut is not so long nor so wide at the hinge. From 0. pepina Hall, it differs in its 

 lower umbo and area, shorter hinge, and smoother surface. It approaches more 

 closely to the form from the Potsdam sandstone of the West, figured by Professor 

 Hall, l)ut not named, except as a Strophoinnia or StropJiodonta." <( 



"Preliminary Notice of the Fauna of the Potsdam Sandstone. Sixteenth Rep. 

 N. Y. State Cab. Nat. Hist., pi. vi, fig. 22. 



