666 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
SPHENOTRETA SULCATA Cooper, new species 
Plate 124, C, figures 18-23 
Shell small, triangular in outline and with the maximum width between the 
middle and the anterior margin; apical angle 70-80°. Anterolateral margins 
fairly narrowly rounded; anterior margin truncated. Anterior commissure mod- 
erately deeply sulcate in the young but nearly rectimarginate in adults. Costate; 
costae narrowly rounded and closely crowded, 18 or 19 on an adult. 
Pedicle valve deeper than the brachial valve and moderately convex in lateral 
profile and with maximum convexity in the anterior half; anterior profile strongly 
convex, with the greatest convexity in the middle. Umbonal region flattened. 
Median region swollen, the swelling forming a narrowly elevated region in the 
posterior half; front third somewhat flattened and bent in the direction of the 
brachial valve; flanks gently convex and with steep lateral slopes. Foramen 
elongate-oval; dental plates short. 
Brachial valve more broadly oval than the pedicle valve, less deep and less 
convex than the pedicle valve. Anterior profile broadly and moderately convex 
but with median region concave. Umbo sulcate ; sulcus moderately deep through- 
out the valve length. Flanks moderately swollen and with precipitous lateral 
slopes. 
Measurements in mm.—Holotype, length 3.6, brachial length 3.1, midwidth 
3.0, greatest width 3.1, thickness 1.4. 
Types.—Holotype: 117190a; figured paratype: 117190b; unfigured paratypes: 
117190c,d. 
Horizon and locality—McLish formation (bed 45) in Oklahoma: On West 
Spring Creek, sec. 6, T. 2 S., R. 1 W., Murray County. 
Discussion.—This species is characterized by its numerous closely spaced and 
narrowly rounded costae, a moderately deep sulcus, and oval outline. The species 
is closest to S. acutirostris (Hall) but differs in having a shallower sulcus, more 
closely spaced costae, and broader outlines. It differs in these same respects 
from S. cuneata. 
DORYTRETA Cooper, new genus 
(Greek dory, spear; tretos, opening) 
Similar to Sphenotreta externally but with the sulcus reverting to a fold an- 
teriorly. Foramen elongate oval and with margin thickened; deltidial plates 
small, triangular. 
Pedicle interior with short divergent dental plates. Brachial interior with 
small triangular socket plates divided by a wide space; crura short, bent abruptly 
toward the pedicle valve. 
Genotype.—Dorytreta bella Cooper, new species. 
Discussion.—This genus strongly resembles Hallina, but no trace of a spire 
was seen in nearly a dozen specimens prepared to show the interior. Inasmuch 
as the interiors are filled by translucent calcite, a loop if present should have been 
detected. The genus resembles Sphenotreta but differs in not having the deep 
