68 
IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
strong, lamellose, concentric growth-lines. Beak acutely angular, 
produced 1 to 2 mm. beyond that of the opposite valve, and but 
slightly recurved; delthyrium large, triangular, and partly occu- 
pied by the beak of the brachial valve. Mesial sinus originates 3 
to 5 mm. anterior to the beak, and is occupied by two to four 
strong, simple plications, the average number being three. 
Brachial valve more convex than the pedicle in adults ; approxi- 
mately equal in young. Postero-lateral slopes regularly curved 
without vertical flattening ; antero-lateral slopes of adult specimens 
abruptly convex, with numerous growth-lines. Mesial fold broad 
and flattened in young specimens; in adults such as the holotype 
it is high and rounded. In all specimens examined the fold bears 
four plications, regardless of aberations on the sinus. The sur- 
face of both valves is marked by fourteen to eighteen strong, 
simple, angular plications which are separated by narrow, angular 
furrows. Near the anterior margin, in adult specimens, there are 
abundant strong, lamellose, concentric growth-lines, which cause 
a pronounced anterior flattening. 
This species is probably the one which Nettleroth figures as R. 
inaequivalva (Cast.). It is associated with R. mercerense of 
this paper, but differs from that species in form and proportion, 
particularly in that the young R. kentuckiense never attains the 
cuneiform shape, the postero-lateral flattening, and the high blade- 
like plications that characterize R. mercerense. 
Formation and Localities : Black river, Danville, Burgis, Paris, 
and Curdsville, Kentucky. 
Holotype, No. 27,445 ; paratypes, No. 27,446, Walker Museum. 
RHYNCHOTRBMA KENTUCKIENSE VARIANS, var. nov. 
Plate I, figs. 1-5 • i{ { ' . 
Shell smaller than medium, longer than wide, subovate to sub- 
triangular in outline, with postero-lateral margins slightly flat- 
tened vertically. Dimensions of two specimens, the first of which 
is the holotype: Length, 10.1 mm., and 11.2 mm.; width, 11.7 
mm. and 12.1 mm. ; thickness, 7.8 mm. and 8 mm. ; width of sinus, 
6 mm. and 6.6 mm. 
Pedicle valve shallow, convex in the unbonal region, but flat- 
tened on the slopes; beak prominent, slightly incurved and pro- 
duced 1 to 1.5 mm. beyond that of the opposite valve. Mesial 
sinus originates about 3 mm. anterior to the beak, and bears 
