742 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
margin broadly rounded. Costellae narrowly rounded, distant, numbering about 
3 to the millimeter at the anterior margin. 
Pedicle valve unevenly convex in lateral profile, the posterior third to half 
somewhat flattened but the anterior half moderately convex; anterior profile 
broadly convex with the median ridge forming a somewhat obtusely angulated 
crest. Umbo narrowly convex, the convexity produced anteriorly to about the 
middle as a low and narrow fold which disappears in the somewhat swollen an- 
terior ; flanks bounding fold slightly convex, gently depressed below the fold; 
anterior third of median region and flanks gently bent and somewhat swollen to 
form a moderately long and steep anterior slope. Interarea moderately long, 
strongly apsacline. 
Brachial valve gently concave in lateral profile and with the maximum depres- 
sion just anterior to the middle and located at the sloping anterior face of the 
inner median ridge. Umbo sulcate, the sulcus produced anteriorly nearly to the 
anterior margin as a widening but shallow depression. Flanks bounding sulcus 
nearly flat. Cardinal extremities flattened. Interarea short, anacline. 
Measurements in mm.—Holotype, length 8.6, brachial length 8.0, midwidth 
12.1, hinge width 13.7+, thickness 1.9, height 2.4. 
Types.—Holotype: 117562e; figured paratypes: 117562b,f-i; unfigured para- 
types : 117562a,c,d. 
Horizon and locality—Upper Pogonip group in Nevada: In Ikes Canyon 
1 mile above the entrance, Toquima Range, Roberts Mountains (1°) Quadrangle. 
Discussion.—No other species of this genus is known to which this species may 
be compared. The exterior form and profiles of the genus are like a number of 
other genera. This genus on its exterior could be mistaken for Sowerbyites but 
as far as known is not externally lamellose and has a flatter brachial valve than 
the members of that genus. It might be confused with Palaeostrophomena, but 
the members of that genus usually have convex brachial valves. 
Family SYNDIELASMATIDAE Cooper, new family 
Plectambonitacea having an exterior like that of Sowerbyella but cardinal 
process simple and not fused to the brachiophores. 
SYNDIELASMA Cooper, new genus 
(Greek syn, together; di, two; elasma, plate) 
Shell attaining 4 inch in width, thin, compressed, concavo-convex, semiellipti- 
cal in outline; sowerbyelloid in profiles and general appearance; hinge forming 
the widest part of the shell; cardinal extremities auriculate; ornamentation dis- 
tantly costellate. Posterior margin with oblique wrinkles. Pseudopunctate. 
Pedicle interior without dental plates; muscle area lightly impressed. Inter- 
area short; pseudodeltidium much reduced or absent. Other details not known. 
Brachial interior with sockets small, moderately deep, bounded by slender 
brachiophores produced into thin, sharp points; cardinal process a short bulb 
when viewed from the side of the pedicle valve; posterior of bulb marked by 3 
