126 CAEBONIFEKOUS PERIOD. 



Width and length of a large specimen, each thirteen millimeters ; but 

 the average size is nearly one-thu-d less. 



This little shell belongs to a section of the genus Orthis, of which 0. 

 MicMlini may be cited as' the type, and of which 0. Pecosii is the only 

 representative known to me in American strata above the Subcarboniferous. 

 It is very constant in its specific characters, and has a wide geographical 

 range. Professor Marcou's type-specimens were obtained from New Mexico, 

 but it is not an unconunon shell in the Middle and Upper Coal-Measures 

 of Illinois, Iowa, Missouri, Kansas, and Nebraska. There is a small species 

 of Orthis in the Keokuk limestone (Subcarboniferous) of Iowa and Illinois, 

 which is very closely related to this one. If it is not identical with 0. 

 Pecosii, the range of the latter is probably confined to strata of the Carbon- 

 iferous period alone. 



Position and locality. — Strata of the Carboniferoixs period ; near Santa 

 F6, New Mexico. 



Genus MEEKELLA White and St. John, 1867. 

 Meekella striatocostata Cox, sp. 



Plate IX, fig. 4 a, b, c, d, and e. 



Plicatula striatocostata Cox, 1857, Geol. Eeport Kentucky, iii, 568. 



Orthisina Shumardiana Swallow, 1858, Trans. St. Louis Acad. Sci., i, 183. 



Orthisina Missouriensis Swallow, 1858, ib., 219. 



Streptorhynchus pyramidalis Newberry, 1861, Exp. Exped. Col. Eiver, Paleont., 126. 



Streptorhynchus occidentalis Newberry, 1861, ib,, 126. 



Orthis striatocostata Geinitz, 1866, Carbonformat. und Dyas iu Nebraska, 48. 



Meekella striatocostata White and St. John, 1867, Trans. Chicago Acad. Sci., i, pt. i, 120. 



Meekella striatocostata Meek, 1872, U. S. Geol. Surv. Nebraska, 175. 



Shell variable in size and shape, indistinctly trihedi'al in outline ; both 

 valves becoming gibbous at full adult age ; hinge-line generally much 

 shorter than the greatest breadth of the shell. 



Ventral valve usually more capacious than the other, but sometimes 

 the difference in this respect is slight, deepest near the umbo ; beak almost 

 always more or less distorted by being flattened, depressed, bent backward 

 or toward one side or the other, usually toward the dextral side ; area 

 triangular, more or less irregular in consequence of the distortion of the 

 beak; height of area seldom so great as its width at the base, and is often 



