Fauna of the Morrow Group 
177 
near the anterior margin. The dimensions of the holotype, a 
somewhat distorted individual, are: length, ±11 mm.; breadth, 
17.3 mm. ; thickness, 13 mm. ; width of sinus in front, 11 mm. 
Pedicle valve shallow, gently convex in umbonal region, flat- 
tened in front of the umbo and deflected very abruptly to the 
lateral and anterior margins ; beak rather broad and closely in- 
curved above that of opposite valve ; mesial sinus originating in 
front of the middle of the valve as a broad, shallow, concave 
depr-ession which is distinct from the lateral surface of the 
valve only near the abrupt anterior deflection of the shell where 
it is depressed below the anterio-lateral extremities of the flat- 
tened portion of the valve, produced anteriorly in a broad flat- 
tened lingual extension whose surface lies in an acute angle to 
the plane of the valve ; plications simple, rounded or subangular, 
originating at the beak, in the holotype six occupying the sinus, 
and flve or six each lateral slope. 
Brachial valve much deeper than the pedicle, the umbonal 
region somewhat flattened, the umbonal slopes steep, becoming 
more abruptly differentiated anteriorly, the greatest convexity at 
about the mid-length of the valve, the surface sloping from that 
point to the beak with an increasing curvature and to the anterior 
margin with an abrupt laterally flattened slope, laterally from 
the median line the surface is gently convex at first and then 
curves abruptly to the margins; the beak broad and incurved 
beneath that of opposite valve; the mesial fold differentiated 
only near the line of abrupt deflection at the summit of the 
anterior slope where it is slightly elevated above the anterio- 
lateral extremities; plications as in opposite valve with seven 
occupying the mesial fold and about flve each lateral slope of 
the holotype. 
Minute surface markings not observable in the material at 
hand, the shell structure minutely punctate. Internal characters 
unknown. 
Remarks. The material upon which this species is based con- 
sists of but two specimens, one a mere fragment and the other 
slightly distorted. It is, however, quite distinct from the previ- 
ously described members of the genus. In comparison with R. 
carhonaria McChesney (—R. illinoisense Worthen) the smaller 
number and greater size of the plications, as well as the mark- 
