oraoin.] • DESCRIPTIONS OF SPECIES. 73 
having come from the flint-bearing limestones of the grupo superior, 
notwithstanding the flinty character of their fossilization and a pos- 
sibilit}^, at least, that both species may have come from some minor 
intercalation or segregation of siliceous matter in either the Ciene- 
guita or the Alamitos beds. 
Lucina potosina var. metrica var n. 
PL XI 11, figs. 6-10. 
Lucina metrica Cragin, 1898, Jour. Geol., vol. 5, p. 817. 
Lucina potosina ? Cragin, 189S, loo. cit., p. 817 (footnote) and p. 819. 
Shell equivalve, compressed or little inflated, flattish-lenticular, 
suborbicular, yet tending somewhat to a four- or five-sided outline, 
usually a little transverse, but often not markedly so, sometimes 
about equilateral, for the most part slightly, and in exceptional cases 
considerably, inequilateral; valves thin; postero-dorsal region some- 
what especially compressed in contrast with the gently inflated uni- 
bonal region in front of it, the concavity here being counterbalanced 
by a feebler compression of the antero-dorsal region; beaks very 
small and depressed, subcentral, usually placed a little back of the 
middle; surface ornamented with widely and regularly interspaced, 
strongly and abruptly elevated, linear-compressed, concentric costellse. 
The spaces between the latter are about six times as wide as the 
costellae themselves, flat, marked only with numerous and fine 
crowded, concentric growth lines; and are remarkable for the regu- 
larity of their breadth, which increases distally with almost imper- 
ceptible increments, but shows occasional variations. 
Measurements. — Height, 34 mm.; length, 35 mm.; breadth, 10 mm. 
in a specimen of the commoner and intermediate size, much smaller 
specimens being similarly proportioned. For the same dimensions, 
an example of average adult size and an exceptionally large one 
give, respectively, 41, 45, 14, and 54, 59, 16 mm. 
Occurrence. — A mile and a half east of Malone station. One of the 
more abundant fossils there occurring, but less common than the 
Pleuromya inconstans. More than 200 specimens are represented. 
A few examples were also obtained in foothills at the northwest end of 
Malone Mountain, about 2 miles east-southeast of Finlay station, 
and west of the mountain, 2 miles north of its southern end. 
The height of the shell in L. potosina, var. metrica is commonly 
from 90 to 97 per cent of the length, which, however, it may occasion- 
ally equal. The breadth averages less than 30 per cent of the length ; 
in 10 measured specimens it averages 29.46, the minimum being 25.5, 
and the maximum being 32 per cent. The corresponding ratios given 
by Castillo and Aguilera for typical potosina are height as percent- 
age of length 0.79 to 0.85, and breadth as percentage of length 0.47 
to 0.48. 
