NO. 1395. CAMBRIAN BRACHTOPODA—WALCOTT. 301 



The transverse dorsal valve has a strong median depression, w hich 

 starts on the umbo and widens out rapidly toward the front. The sur- 

 face of the shell is marked by concentric ridges and lines of growth 

 and very line concentric strife. 



This species differs from other described forms in the strong median 

 depression of the dorsal valve. 



Form at! on and locality. — Middle Cambrian. Limestone above <:\ iiart- 

 zite, Mount Nebo Canyon, 8 miles southeast of Mona, Utah. 



ACROTRETA NOX, new species. 



This minute shell, 1 mm. in length, is characterized bv its low ven- 

 tral valve, nipple- like apex, and apparentl}^ smooth surface. 



Formation and locality. — Upper Cambrian. Argillaceous shales in 

 St. Croix sandstone, 8 miles east of Baraboo, Wisconsin. 



ACROTRETA PACIFICA, new species. 



Ventral valve a high cone, with the apex a little in advance of the 

 posterior margin: l)ase circular, with the exception of a slight flatten- 

 ing on the posterior side; apex acute and pointing upward; false area 

 only a slight flattening of the shell from the apex to the margin. The 

 largest ventral valve has a diameter and height of about 1.5 mm. Sur- 

 face marked b}' tine concentric stria? that continue without noticeable 

 deflection across the false area. 



Ohservations. — Onl}^ two specimens of the ventral valve of this 

 species occur in the collection. One of these has a slightly transverse, 

 broadh" oval base, and a more definite false area. The species is closely 

 related to Acrotreta idahoensis <dta^ except that the ventral valve is not 

 quite as elevated. 



Formation and locality. — Middle Cambrian. Chang Hsia limestone 

 at Yen Chuang and 3^ miles southwest of Yen Chuang; 3 miles south- 

 west of Yen Chuang in limestone nodules at base of green shale phase 

 of the Chang llsia oolitic formation; Shantung, China. 



Collections of Mr. Eliot Blackwelder, Carnegie Institution of Wash- 

 ington, expedition to China. 



ACROTRETA SHANTUNGENSIS, new species. 



Shell small. Ventral valve a low cone with the apex a little forward 

 of the posterior margin, Avhich is slightly flattened; apex minute, 

 directed backward and projecting slightly over the faintly defined 

 false area. The cast of the interior shows that the apical callosit}^ was 

 rather large and the main vascular sinuses well defined on each side of 

 it; the cardinal scars ai"e small and not prominent. 



Dorsal valve slightly convex; apex marginal; surface marked by a 

 rather broad shallow median depression that begins on the umbo and 

 gradually widens toward the front margin. The interior of the dorsal 

 Proc. N. il. vol. xxviii— 04-. 20 



