804 CAMBEIAN BKACHIOPODA. 



This species differs from SyntropTiia harahuensis (Winchell) in its smaller size, less trans- 

 verse outline, and more pronounced mesial depression on the ventral valve. Some specimens 

 of the dorsal valves of the two species ai-e very similar. These are shown by Plate CII, 

 ficrures If and 2c. Some of the shells approach certain forms of S. calcifera, but the larger 

 number are less convex and more transverse and with more obtuse cardinal angles. A shell 

 closely allied to S. primordialis , and apparently identical, occurs in the limestone of the Keagan 

 sandstone of Oklahoma. Only the exterior is known. 



Formation and locality. — Upper Cambrian: "St. Croix sandstone" at the following localities: (99) Minneiska 

 (Miniska), on Mississippi River, near the line between Wabasha and Winona counties; (339d) Taylors Falls, Chisago 

 County; (84a) at River Junction, Houston County, 20 miles (32.2 km.) below Dresbach, and (339g) just below the 

 Dikellocephalus minnesotensis beds, near Winona, Winona County; all in Minnesota. 



(328k) "St. Croix sandstone" at Roche a Cris Bluff, Adams County, Wisconsin. 



(9u) About 195 feet (59.4 m.) above the porphyry contact in the limestones of the Reagan sandstone, in SE. J 

 NE. i sec. 2, T. 4 N., R. 13 W., 15 miles (24.2 km.) northwest of Fort Sill, Comanche County; and (12n) limestones 

 •of the Reagan sandstone (in the section 7 miles (11.2 km.) north of Springer this horizon is about 240 feet (73 m.) above 

 the porphyry contact and 40 feet (12 m.) below the Arbuckle limestone), NW. J sec. 1, T. 2 S., R. 1 E., Ardmore quad- 

 rangle (U. S. Geol. Survey), Carter County; both in Oklahoma. 



(188) Limestones on north side of Tepee Creek, near the road from Sheridan to Dome Lake, Bighorn Mountains, 

 Sheridan County, Wyoming. 



Syntrophia primordialis argia Walcott. 



Plate CII, figure 3. 

 Syntrophia primordialis argia Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 293. (Characterized as below as a new 

 variety.) 

 This specimen is separated as a variety of SyntropTiia primordialis (Whitfield) on account 

 of its less convexity and more shallow median sinus, which has three rudimentary plications in it. 

 Formation and locality. — Tipper Cambrian: (97) "St. Croix sandstone" at Reads Landing, foot of Lake Pepin, 

 Wabasha County, Minnesota. 



„ Syntrophia rottjndata Walcott. 



Plate cm, figures 4, 4a-e. 



Syntrophia rotundata Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 293-294. (Described and discussed as below 



as a new species.) 



General form rotund, unequally biconvex; hinge line short. Surface marked by fine, 

 concentric strisB and low, imbricating varices of growth. The largest shell observed has a 

 width of 13 mm.; length, 11 mm. Ventral valve convex at the umbo and beak, but depressed 

 toward the front by a strong, broad, and deep median sinus. Area low, short, and divided 

 midway by a relatively large triangular delthyrium. Transverse sections of the umbo near 

 the beak show the cross section of a well-developed spondylium supported on a median septum. 

 Dorsal valve about as convex as the ventral on the umbo, but the strong, broad median fold 

 makes it strongly convex. Cross sections at the umbo show a spondylium and supporting 

 septum very much like that in the ventral valve. 



Observations. — This species is taken as the Cambrian representative of the genus Syri^ 

 tropJiia. It has a spondylium in each valve supported by a median septum in the same manner 

 as in the Ordovician type of the genus S. lateralis. Its rotund form, deep ventral sinus, and 

 strong dorsal fold serve to distinguish S. rotundata from other species of the genus. 



Formation and locality.— Upper Cambrian: (14k) Limestone on Wolf Creek, 15 miles (24.2 km.) west-south- 

 west of Sheridan, Bighorn Mountains, Sheridan County, Wyoming. 



Syntrophia ? tjnxia Walcott. 

 Text figure 74, page 805. 

 Syntrophia? unxia Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 108-109, PI. X, fig. 10. (Discussed 

 as below as a new species. Fig. 10 is copied on p. 805 as fig. 74.) 



This species is represented by a single specimen of the ventral valve from which the 

 shell has been removed by weathering. The cast of the spondylium shows it to have been 



