960 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
originating at anterior end of diductors, close and separated by a low, narrow 
median ridge. Dental plates obsolete. 
Brachial valve moderately convex in both profiles; fairly strongly sulcate at 
the beak and umbo but sulcus nearly disappearing near the middle. Brachio- 
phores a stout blade with a poorly defined crus given off from the inner end; 
sockets deep; notothyrial cavity choked by excess shell supporting the inner 
sides of the brachiophores; cardinal process with short, stout shaft and lobate 
myophore, the middle lobe of the myophore forming a high, narrow ridge. Me- 
dian ridge low and stout, separating the small adductor scars. 
Genotype—Eremotrema biconvexum Cooper, new species. 
Discussion.—This genus has obvious affinities with Paucicrura in the cardi- 
nalia of the brachial valve, but the shape, outline, and interior of the pedicle 
valve present important differences from that genus. Eremotrema is character- 
ized by its subequally convex valves, long interareas on both valves, nonfascicu- 
late costellae, rhomboidal muscle area, and stout, encrusted cardinalia. 
Eremotrema differs from Paucicrura in the fairly strongly biconvex outline, 
that of Paucicrura being gently biconvex to planoconvex. The major difference 
is inside the pedicle valve, that of Eremotrema not having well-defined dental 
plates, ponderous teeth, and a muscle field with rhomboidal outline. The diductor 
scars in the muscle field are elongate and terminate with their anterior ends close 
together, which is quite unlike that of Paucicrura in which the anterior end of the 
muscle field is bilobate. The pallial trunks of Eremotrema are unlike those of 
Paucicrura but are suggestive of Paurorthis in being parallel and separated only 
by a low median ridge. The vascula media of Paucicrura are usually strongly 
divergent, not convergent as in Eremotrema. 
Inside the brachial valve the cardinalia are like those of Paucicrura but are 
more exaggerated. Moreover, all the structures are thickened. The brachio- 
phores are supported by shell substance deposited on their inside. The cardinal 
process, unlike that in Paucicrura, protrudes from the notothyrium and projects 
beyond the margins of a long interarea. 
EREMOTREMA BICONVEXUM Cooper, new species 
Plate 146, G, figure 62; plate 160, E, figures 31-45 
Subquadrate to subcircular in outline ; sides gently rounded to nearly straight; 
widest just anterior to the middle; anterior margin broadly rounded; antero- 
lateral extremities narrowly rounded; anterior commissure rectimarginate. 
Valves subequally convex; surface costellate, costellae fine, rounded, separated 
by spaces equal in width to the width of the costellae ; 3 costellae to the millimeter’ 
at the front margin and 4 costellae to the millimeter on the sides. 
Pedicle valve gently convex in lateral profile; anterior profile with the median 
region elevated and long, moderately steep slopes to the margins. Beak small, 
well defined, incurved; umbo narrowly convex, forming a subcarinate fold that 
disappears at about the middle of the valve. Median region swollen; lateral 
slopes gently convex. Interarea nearly orthocline, gently curved, long; del- 
thyrium long and narrow. 
