220 
Kirtley F. Mather 
This common Coal Measures form is represented in the Mor- 
row group by four specimens from the Hale formation, which 
cannot be distinguished from certain of the variants ordi- 
narily identified as belonging to this species. The characteristic 
curvature of the cardinal portion of the umbonal ridge, the 
slight flattening and extension of the shell in front of the umbo 
above the middle, and the concentric lamellose markings of the 
surface are well shown. The umbonal ridge is, perhaps, not 
so sharply elevated as in the more typical specimens but this 
and other minor variations in the contour of the valves are 
presumably to be explained by the stunted growth of the Mor- 
row forms, none of which attained more than 30 mm. in its 
longest diameter. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Station 136). 
Myalina cuneiformis Gurley? 
Plate XV, figure 3. 
1883. Myalina cuneiformis. Gurley, New Garb. Foss., Bull. No. 1, p. 4. 
Upper Carboniferous: Ouray, Colorado. 
1903. Myalina cuneiformis. Girty, Prof. Paper, U. S. Geol. Surv., No. 
16, p. 420, pi. 8, figs. 14-17. 
Carboniferous beds: Ouray, Colorado. 
Weber limestone: Crested Butte district, Colorado. 
An internal cast of a Myalina from the Kessler limestone re- 
sembles so closely the type and other specimens of M. cunei- 
formis that it does not seem practicable to differentiate it from 
that species. The Kessler valve displays the same cuneate out- 
line approaching a triangular form as do the Colorado shells. 
The umbonal ridge is even more sharply elevated than in the 
type material and the umbonal slopes are correspondingly 
steeper. The angle between cardinal and anterior margins is 
a very acute one and the former is not sharply differentiated 
from the posterior margin into which it curves below. The 
umbonal ridge in the ventral portion of the valve maintains 
its position close to the anterior margin with the anterior slope 
of the valve surface much more abrupt than the posterior slope. 
The material at hand represents a smaller form than is com- 
mon for this species but a stunted growth seems to be charac- 
teristic of many of the Morrow pelecypods. Its dimensions are: 
