Fauna of the Morrow Group 
195 
RHYNCHOSPIRID^ 
Genus HUSTEDIA Hall & Clarke 
Hustedia brentwoodensis n. sp. 
Plate XIII, figures l~3c; text figure 5. 
Description. Shell small, sub-trigonal in outline, valves sub- 
equally convex, nearly twice as long as wide, the greatest width 
in front of the middle. The dimensions of three perfect speci- 
mens are: length, 8.4 mm., 6.8 mm., 4.9 mm.; width, 4.8 mm., 
4.3 mm., and 3.6 mm. ; thickness, 5.3 mm., 4.2 mm., and 3.1 mm. 
Pedicle valve strongly convex, the surface curving regularly 
from the beak to the anterior margin but sloping almost ver- 
tically from the umbonal region to the lateral margins, the 
greatest convexity occurring at the mid-length of the valve ; 
anterior margin almost straight, rounding abruptly into the 
0 9 Q 0 
Fig. 5. A series of four cross sections of the rostral portion of the shell of 
Hustedia brentwoodensis (x ?J). 
nearly straight lateral margins which converge toward the 
beak; beak small, pointed and incurved above that of opposite 
valve, truncated by a small circular foramen; surface marked 
by 10 to 14 simple, sub-angular plications, originating on the 
beak, 4 or 5 occupying the middle portion of the shell and inter- 
secting the anterior margin, and 3 or 4 on either lateral slope. 
Brachial valve about as convex and having the same shape 
as the pedicle valve except that it is somewhat shorter; beak 
closely incurved beneath that of opposite valve; plications simi- 
lar to those of pedicle valve. 
Internally, the inner surface of the pseudodeltidium bears 
a split tube attached by its closed side to the deltidial plates 
with its open side directed toward the interior of the shell as 
shown in figure 5. The umbonal cavity of the brachial valve is 
occupied by a median septum. Shell structure finely puctate. 
Remarks. The appearance of the plications of the surface 
of this shell presents quite different aspects when exfoliated 
from that seen when the shell surface is preserved. In the 
