Fauna of the Morrow Group 
221 
greatest diameter, from beak to ventral margin, 10.7 mm. ; 
breadth, normal to umbonal ridge, 5.3 mm. ; length of hinge- 
line, 5.3 mm. ; convexity of valve, about 2,5 mm. 
Horizon and locality. Kessler limestone: East Mountain, 
Fayetteville, Arkansas (Station 209). 
Myalina orthonota n. sp. 
Plate XVf figure Jf. 
Description. Shell of medium size, sub-oval in outline, very 
inequilateral; hinge-line comparatively short, little over one- 
third as long as greatest diameter of shell, that from beak to 
posterio- ventral extremitj/-; anterior and cardinal margins con- 
verging at an angle of about 50°, anterior margin nearly 
straight, posterior margin gently convex, the two approximately 
parallel throughout the middle half of the shell and rounding 
gracefully and sub-equally into the strongly convex ventral mar- 
gin; the umbonal ridge nearly straight with only a slight for- 
ward curvature very close to the beak, strongly elevated above 
with steep umbonal slopes on either side, less prominent below 
where it merges with the curvature of the valve; valve not 
produced in front of the umbo, the umbonal slope in front 
continuing with scarcely lessened declivity to the line of union 
between the valves; surface marked by ^close-set, lamellose, 
concentric, growth lines. 
The dimensions of the holotype are: greatest diameter, from 
beak to posterio- ventral extremity, 16.2 mm. ; length of hinge- 
line, 6.5 mm. ; breadth, normal to umbonal ridge, 7.6 mm. ; con- 
vexity of valve, about 3.5 mm. 
Remarks. This species is characterized by its comparatively 
straight umbonal ridge and anterior margin as well as by its 
proportionately long diameter from beak to opposite extremity. 
Its nearest affinities are M. pernaformis and M. perattenuata, 
from which it may be distinguished by the characteristics just 
mentioned. In the Hale formation all the individuals are, as 
well, comparatively small, the largest having an oblique diameter 
of only 26 mm., but in the Kessler limestone is a much larger 
form, only one specimen of which is at hand, which is question- 
ably referred to the same species. The umbonal portion of this 
individual is missing and it is not possible to identify it with 
