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ACEPHALA. 433 
Length an inch and a quarter; height three-fourths of an inch. 
breadth three-eighths of an inch. 
Inhabits Wallawalla, Oregon. Dr. Pickering. 
A small and rather remarkable species, like a miniature U. compla- 
natus. It is more equilateral, has no dorsal angles, the dorsal and 
ventral margins diverge more, and the cardinal teeth are more oblique. 
It also resembles U. paludicolus, from Florida, but is smaller. 
Figures 544, 544 a, 5443, three views of the shell. 
ALASMODONTA FALCATA (Gould). 
T. transversa, vald? inequilaterals, falcata, antice rotundata, postice 
deflecta, acuta, marginibus subparallelis ; disco sub-excavato ; umbo- 
nibus obtusis, erosis ; epidermide subfusco ; dente cardinal in valva 
dextra elevato, trigono, in valva sinistra longitudinal, compresso-trt- 
angulart ; margarita purpurascente ; limbo argenteo. 
Alasmodon falcata, Goutp; Proceed. Boston Soc. Nat. Hist., m1. 
294. Nov. 1850. Expedition Shells, 87. 
SHELL large, transversely elongated, inequilateral, falcate, dorsal 
margin gently curved; basal margin nearly parallel with it: anterior 
extremity rounded; posterior deflexed, produced, rather acute; disk 
somewhat excavated: beaks at the anterior fourth, slightly raised, 
eroded: epidermis dead sooty-black: surface concentrically waved 
with crowded, but strongly-marked, almost lamellar lines of growth ; 
cardinal tooth, left valve, prominent, laterally compressed, pyramidal ; 
with a very small obsolete tooth in front, separated by a pit: a single 
oblique pyramidal tooth in the right valve, with a fissure in front and 
a deep pit behind: nacre pearly at margin, tinted pale copperas or 
peach-blossom colour near the cavity of the beaks, and clouded with 
livid: muscular impressions very profound, anterior one distinct: 
cavity of the beaks considerable. 
Length four inches; height one and a half inches; breadth one inch. 
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