Fauna of the Morrow Group 
201 
by a faint mesial line which seems to be a character of the shell 
structure rather than a depression in the shell surface or it 
may be represented anteriorly by a slight broad downward 
flexure of the line of contact between the valves. 
Brachial valve almost exactly circular in outline, slightly less 
convex than the pedicle valve, transverse curvature ordinarily 
a little stronger than the longitudinal; beak somewhat promi- 
nent, especially in the more mature individuals, because of a 
slight flattening toward either side. 
Surface of both valves marked by faint concentric lines of 
growth which in most cases are nearly as prominent in the 
umbonal region as toward the anterior margin. Shell structure 
fibrous, giving many specimens the appearance of being covered 
with exceedingly fine thread-like striae. 
Remarks. This form— one of the most numerous in the 
Brentwood limestone and abundant in all fossiliferous horizons 
in the Morrow group— is frequently observed at higher and 
lower elevations in the Carboniferous formations of the south- 
ern and western Mississippi valley. It is apparent, as pointed 
out by Girty^h that the great anterior thickening of the shell, 
so conspicuous in White's figures, is due to old age and cannot 
be considered a specific character. Such a thickening is absent 
from the Arkansas forms. It is probable that C. humilis of the 
Madison limestone and the Moorefield shale is conspecific with 
the forms here described. 
Doubtless the smaller individuals included under this title 
embrace also the immature members of some or all of the other 
species of this genus found in the Morrow collections, as their 
specific characters are not acquired until a length of about 5 mm. 
is attained. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Stations 136 and 149). Brentwood lime- 
stone: near Fayetteville, Arkansas (Stations 134, 135, 148, and 
153) ; Sawney Hollow, Oklahoma (Station 210). Kessler lime- 
stone: East Mountain, Fayetteville, Arkansas (Station 209). 
Morrow formation: near Ft. Gibson (Stations 301 and 303), 
and Choteau (Station 302), Oklahoma. 
