144 
Kirtley F. Mather 
delthyrium about twice as high as wide, occupying the middle 
fourth of the area; mesial sinus originating in front of the 
umbo as a broad, shallow, rounded, undefined depression which 
becomes more conspicuous near the anterior margin; surface 
apparently devoid of radiate markings, although this may be 
due to the preservation of the available material, concentric 
growth lines conspicuous especially near the anterior and lateral 
margins of the valve. Hinge teeth large, deflected outward 
from the hinge-line at the base of the cardinal area. 
Brachial valve and internal characters unknown. 
Remarks, s This species, based upon a single pedicle valve, 
is characterized by the strong elevation of the cardinal area. 
It is referred to Rhipidomella with considerable confidence in 
the absence of information concerning the muscular scars be- 
cause of the general outline and appearance of the valve and 
the presence of the pair of large hinge-teeth. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Station 139). 
Rhipidomella pecosi (Marcou) 
Plate VIII, figures 3, 3a. 
1858. Orthis pecosii. Marcou, Geol. N. A., p. 48, pi. 6, figs. 14-14b. 
Mountain limestone: Pecos Village, New Mexico. 
1858. Orthis carbonaria. Swallow, Trans. St. Louis Acad. Sci., vol. 1, p. 
218. 
Middle Coal Measures: Lexington, Missouri. 
18*64. Orthis (sp. undet.) Meek, Pal. Cal., vol. 1, p. 10, pi. 2, figs. 5-5c. 
Carboniferous: Bass’s Ranch, Shasta County, California. 
1872. Orthis carbonaria. Meek, U. S. Ceol. Surv. Nebr., p. 173, pi. 1, 
figs. 8a-c. 
Upper Coal Measures: Rock Bluff and Nebraska City, Nebraska; 
Iowa; Kansas; Illinois. 
Middle Coal Measures: Lexington, Missouri. 
1873. Orthis carbonaria. Meek and Worthen, Ceol. Surv. 111., vol. 5, p. 
571, pi. 25, fig. 4. 
Upper Coal Measures: LaSalle, Illinois. 
1877. Orthis Pecosii. White, U. S. Ceog. Surv. west of 100th Merid., vol. 
4, p. 125, pi. 9, figs. 5a-c. 
Carboniferous: Santa Fe, New Mexico. 
1883. Orthis carbonaria. Hall, 2nd Rep. N. Y. State Ceol., for 1882, pi. 
(7) 37, figs. 1-4. 
Coal Measures: Springfield, Illinois. 
