WHITE. ] CARBONIFEROUS FOSSILS. 137 
shell; but this proportion varies with the varying proportional length 
of the posterior portion of the shell in diiferent examples. Surface 
marked by fine, regular, raised concentric strize, which are obsolete, or 
jess conspicuous, on the postero-dorsal space between the umbonal ridges. 
Cardinal teeth, about twelve on each side of the beak; cartilage pit 
moderately large. Interior markings unknown. 
Leneth of the most perfect example in the collection, 30 millimeters ; 
height from base to beaks, 18 millimeters; thickness, both valves to- 
gether, 12 millimeters. Some less perfect examples indicate a greater 
size and a proportionally greater elongation posteriorly. 
This species is closely related,to the Leda bellistriata of Stevens ; but, 
besides being a larger and more robust form, it differs conspicuously in 
its laterally flattened and longitudinally strongly concave postero-dersal 
area. 
There can be no reasonable doubt that this form is genetically related to 
N. bellistriata Stevens sp.; but the differences here pointed out are evi- 
dently constant, and therefore deserve recognition; and the most con- 
venient form of expressing such recognition is thought to be in the use 
of a separate specific name. 
Position and locality—Mr. Gilbert obtained this species from the up- 
permost of the Carboniferous strata at Wild Band Pockets, Northern 
Arizona, 15 miles southward from Pipe Spring, where it is associated 
with the species last described, and also with all except two of the fol- 
lowing: 
Genus ALLORISMA King 
ALLORISMA? GILBERTI White. 
Plate 33, figs. 9a and b. 
Allorisma? gilberti White, 1879, Bull. U. S. Geol. Sur. Terr., vol v, p. 217. 
Shell rather small, elongate, broadly reflexed when adult, but Jess 
so while young; posterior portion compressed, and apparently slightly 
gaping; valves broadly convex, except posteriorly, where they are ver- 
tically flattened or compressed; nowhere gibbous; antero-dorsal por- 
tion of the shell obliquely truncated downward and forward directly 
from the beaks to the front; front margin rounded with a regular curve 
to the base; basal margin broadly convex; posterior margin short, 
abruptly truncating that portion of the shell downward and _ back- 
ward, forming a distinct but slightly obtuse angle with the dorsal mar- 
gin; the latter margin broadly concave from the beaks to the posterior 
margin; beaks small, placed near the anterior end of the shell, slightly 
elevated, and turned a little forward; surface marked by concentric 
lines and wrinkles of growth. Also, extending from behind the beaks 
to the narrow posterior end of the shell, there are upon each valve two 
narrow, obscure folds, which increase a little in width with their exten- 
sion posteriorly, and have a shallow groove between them. The lower 
one of these folds is the broader, and it ends at the postero.basal angle; 
and the upper, which is involved with the inflexed dorsal border, ends 
at the postero-dorsal angle. 
Length, 29 inillimeters; height, from base to beaks, 12 millimeters ; 
thickness, both valves together, 74 millimeters. 
This shell departs somewhat widely, in general aspect, from the typ- 
ical forms of Allorisma, and it perhaps does not strictly belong to that 
