536 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Brachial valve oval in outline, gently convex in lateral and anterior profiles 
but both profiles with a greater convexity than those of the pedicle valve. Um- 
bonal and median regions gently inflated ; lateral slopes short, moderately steep. 
Fold not defined. 
Measurements in mm.—Holotype, length 5.1, brachial length 4.2, width 4.4, 
hinge width 1.6, thickness 1.6. 
Type.—Holotype: 117091. 
Horizon and locality—Lower Bromide formation (Mountain Lake member) 
in Oklahoma: On the McLish Ranch, sec. 24, T. 1 S., R. 7 E., Johnston County. 
Discussion—This species is characterized by the modest convexity of its 
valves. In this respect it is unlike any other described form. It is closest to 
O. brevirostris but is flatter and has a more attenuated beak. 
When the fold and sulcus of this species are studied with care, a suggestion 
of anterior folding or costation can be seen. The costae are very indistinct and 
can only be detected by examination of the commissure. Inasmuch as this is the 
youngest species of this primitive genus yet seen, this anterior costation might 
indicate a trend in its folding not seen in older forms. It might also indicate 
that the specimen is actually a young form of a costellate triplesid such as 
Oxoplecia. Members of that genus are abundant in parts of the Bromide forma- 
tion of Oklahoma, but all the many specimens examined by the writer show 
the costation originating on the beak. The specimen assigned to Onychoplecia 
is quite smooth over the entire surface except at the commissure. 
ONYCHOPLECIA sp. t 
Plate 121, C, figures 9-12 
Four specimens of Onychoplecia taken from high in the Sevier formation are 
interesting because they represent the youngest specimens of this genus yet found 
and because they indicate another undescribed species. Unfortunately, the speci- 
mens are not adequate on which to base a species. The material consists of one 
immature pedicle valve and two brachial valves which are probably mature. 
The pedicle valve is 5.8 mm. long and 5.1 mm. wide at the widest part which 
is anterior to the middle. The beak angle is about 70°, and the anterior is nar- 
rowly rounded. In profile the maximum convexity is anterior to the middle. 
A trace of a sulcus is to be seen at the anterior, but the shell is so young that this 
adult character had not yet developed. A second pedicle valve is represented by 
an imperfect impression which is clearly adult but has a long and deep sulcus. 
The specimen has some camerellid features, but there is no way to prove the 
true generic affinities. Although one of the brachial valves mentioned below also 
has camerellid features, its interior details indicate that it is a triplesid. 
Two brachial valves are present in the collection. One of these (117096b) has 
a moderately prominent fold which is wrinkled by 3 costae. This is an unusual 
feature for these triplesids, but the interior shows no median septum or other 
camerellid features. The second brachial valve (117096d) is clearly a triplesid. 
The shell is nearly smooth and has a subcarinate fold originating near the middle. 
