BRADLEY: BR.\CHIOPODA OF THE MAQUOKETA OF IOWA. 511 



is shallow. The convexity is not such as to suggest their tri\dal name, 

 but it appears from Sardeson's description that this was an allusion 

 to the condition of gerontic specimens. 



Measurements: — -The largest specimen is 12.75 mm. long, 13 mm. 

 in greatest width, and 9 mm. wide at the hinge. A smaller one is 8.75 

 mm. long, 9.25 mm. wide, and 7 mm. wide at the hinge. 



Locality: — A rare species in the Lower Maquoketa at Clermont, 

 Iowa, previously reported only from the same formation at Granger 

 and Spring Valley, jNIinn. 



Plaesiomys subquadeata Hall. 



Orthis subqzmdrata HaU, Pal. N. Y., 1847, 1, p. 121, pi. 32A, fig. 1. Geol. 



Wise. 1862, 1, p. 54, fig. 1, 2. Meek, Pal. Ohio, 1873, 1, p. 94, pi. 9, fig. 2. 



jNliller, Cincinnati quart, journ. sci., 1875, 2, p. 38. White, 2d. Ann. rept. 



Indiana bur. state geol., 1880, p. 484, pi. 1, fig. 3-5. Shaler, Mem. Geol. 



surv. Kentucky, 1876, 1, p. 22, pi. 7. Keyes, Missouri geol. surv., 1895, 



5, p. 60. 

 lOrthis subqiiadrafa Billings, Geol. Canada, 1863, p. 165, fig. 146. 

 Platystrophia subquadrata HaU, 36th Rept. X. Y. state mus. nat. hist., 1884, 



p. 75, pi. 3, fig. 4. 

 Plaesiomys subquadrata Hall and Clarke, Pal. N. Y., 1892, 8, pt. 1, p. 194, 196, 



222, pi. 5A, fig. 17-19. 

 Orthis {Dinorthis) subquadrata Winchell and Schuchert, Geol. Minn., 1893, 3, 



pt. 1, p. 428, pi. 32, fig. 46-50. Whiteaves, Pal. foss. Canada, 1897, 3, 



pt. 3, p. 176. 

 Dinorthis subquadrata Hayes and Ulrich, Folio 95 U. S. G. S. illus. sheet, 1903, 



fig. 7, 8. Cumings, 32nd Ann. rept. Dept. geol. nat. res. Indiana, 1908, 



p. 904, pi. 34, fig. 1-lb. 



Two specimens of this common species were found at Clermont by 

 the writer. This fossil is Axry characteristic of the Upper portion of 

 the Maquoketa shale and is found plentifully at Spring Valley, Minne- 

 sota. These rocks are known to be of about the same age as the 

 jMaquoketa of Iowa, so one must look for the cause of the scarcity of 

 Dinorthis subquadrata in the shales at Clermont. 



This species is more convex than the Orthidae with which it is 

 associated. It has a shallow sinus on the pedicle-valve, but not 

 nearly so pronounced a one as that of its close relative, Dinorthis 

 proavita. One feature separates it conclusively from the other species 

 in this locality; the posterior lateral radiating striae curve so strongly 

 outward that a few of them run out on the cardinal edge before reach- 



