Subcarboniferous Fossils from New Mexico. | 315 
convex from the cardinal line to the front with the greatest convexity 
in the middle. ‘The mesial fold is round and truncated by the exten- 
sion of the sinus in the opposite valve. The beak is scarcely defined, 
and does not project beyond the narrow area which extends the whole 
width of the shell. Ventral valve more gibbous than the dorsal valve, 
and most convex at the posterior third. The mesial sinus is rounded, 
and gradually widens from the beak to the front where it is produced 
so as to truncate the mesial elevation of the opposite valve. The um- 
bo is prominent, and the beak sharp and incurved over the fissure. 
The area extends to the cardinal angles, ts slightly concave, especially 
in the middle part, and is transversely striated. 
The surface on either side of the mesial fold and sinus is marked by 
about six simple rounded plications, which become less prominent as 
they recede from the fold and sinus. The entire surface is covered 
by fine, concentric, imbricating lamellose lines, which become more 
marked toward the front of the shell. 
RHYNCHONELLA TUTA, nN. Sp. 
Plate VII., fig. 11, ventral view; fig. lla, dorsal view; fig. 11b, front view; all natural size. 
Shell small, suboval, greatest width at the anterior third, length and 
width sub-equal, ventral side most convex. 
Ventral valve, gibbous, highly convex in the central region; mesial 
_ sinus, not clearly defined; beak acute and closely incurved upon the 
beak of the opposite valve. 
Dorsal valve, moderately convex; mesial depression, imperfectly de- 
fined; beak acute and incurved beneath the beak of the opposite valve. 
Surface marked by about eighteen fine, sharply angular, simple pli- 
cations on each valve. 
This species is founded upon four specimens, each of which is more 
or less injured. The lateral and front margins appear to be in one 
plane. There is uo species in rocks of the same age with which it is 
necessary to make any comparison. 
