744 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
to produce a low but short fold that may die out near the middle but in some 
specimens continues more or less prominently to the front margin ; flanks slightly 
convex, long and low. 
Brachial valve nearly flat to gently concave; greatest concavity just anterior to 
the middle; umbo sulcate, the sulcus continuing and widening anteriorly to form 
a prominent but shallow depression that terminates at the anterior margin. 
Flanks bounding sulcus flat to gently concave; cardinal extremities flattened. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness Height 
Fiolotype esics sees ce 8.8 8.5 13.9 15.6 ? 1.0 1.8 
Paratype (117565c) ... 9.6 9.0 13.7 ? 1.4 1.9 
Types—Holotype: 117565g¢; figured paratypes: 117565a-e,h,i; unfigured 
paratypes: 117563a-c, 117505f,j,k. 
Horizon and locality—Pogonip group (upper 30 feet) in Nevada: About 
t mile above the entrance to Ikes Canyon, Toquima Range, Roberts Mountains 
(1°) Quadrangle. 
Discussion —No other species of this genus is now known to which Syndie- 
lasma biseptatum may be compared. This species occurs with Taphrodonta 
parallela which it resembles in superficial characters. Taphrodonta, however, is 
a smaller, differently proportioned, and stouter shell than that of Syndielasma so 
that the two should not be confused. 
Family LEPTELLINIDAE Ulrich and Cooper, 1936 
Concavo-convex, costellate Plectamobonitacea with low and expanded cardi- 
nal process plate and more or less well developed visceral disk. 
Subfamily HEsPEROMENINAE Cooper, new subfamily 
Leptellinidae having rudimentary visceral disk. 
HESPEROMENA Cooper, new genus 
(Greek hespera, west; mene, crescent) 
Shell with outline and profiles of Leptellina; strongly geniculated near the 
middle ; surface marked by alternating fine and coarse costellae. Pseudopunctate. 
Pedicle valve without dental plates and with deep delthyrial cavity; muscle 
field large; diductor scars long and slender. Other details not known. Pseudo- 
deltidium long, moderately convex. 
Brachial valve with short, stout brachiophores ; median septum low ; cardinal 
process forming a small suboval shelf overhanging the notothyrial region ; cardi- 
nal process with median septal ridge and 2 lateral lobes separated by a narrow 
groove. Visceral area marked by a thickened line. Entire surface strongly 
pustulose. 
Genotype.—Hesperomena leptellinoidea Cooper, new species. 
