126 THE CHICAGO ACADEMY OF SCIENCES. 
long, very narrow and curved, and extends to a point about the 
center of the hinge plate; the lower toothis moreorless pyram- 
idal, elevated and somewhat Jamellar. In the second type 
(Fig. 12) the single right cardinal is very much arched, swollen 
at both ends of the arch, the right arm being the longer. The 
upper left cardinal is long and thick, elevated, lamelliform, and 
extends nearly to the ventral border of the hinge plate; the 
second left tooth is more or less rounded, pyramidal, elevated, 
and extends well toward the dorsal margin of the hinge plate. 
Distribution: North and South America, Europe, New 
Zealand, India, Iceland, Greenland. 
KEY TO SPECIES OF PISIDIUM. 
a. Umbones placed very near the center of the shell, which 
is clongated:oval.in form... sic «melon oe clerir cease Crier abditum 
1. Differs from ‘‘a” in being smaller, shorter and 
higher, the beaks less full, and the sculpture 
finer‘and more: polishensi50 7. ks ioc ceee + tee ate politum 
b. Umbones placed near the posterior end of the shell. 
1. Shell very oblique, posterior end short, rounded, 
anterior end long, obtusely triangular, larger 
fhan)(2)i6.4..-c. Skee See ce ea isle Jee re virginicum 
2. Shell trigonal and triangular, more compressed 
than (1), posterior end short and rounded, an- 
terior end much produced, beaks somewhat 
elesated and ‘COntpressed: vo. scsi ew eben oe compressum 
47. Pisidium Abditum Haldeman, pl. xxviii., fig. 1. 
Pisidium abditum HALDEMAN, Proc. Phil. Acad., p. 53, 1841. 
Cyclas major C. B. ApaAms, Proc. Bost. Soe. N. H., p. 48, 1841. 
Pisidium tenellum Gouip, Agassiz, Lake Superior, p. 235, 1848. 
Pisidium minus Stimpson, Shells of New England, p. 16, 1851. 
Pisidium obscurum PRiME, Proc. Bost. Soc. N. H., Vol. IV., p. 161, 1851. 
Pisidium kurtzi Prime, |. c, p. 162, 1851. 
Pisidium zonatum Prime, l. c., p. 162, 1851. 
Prisidium rubellum Prime, 1. c., p. 163, 1851. 
Pisidium regulare PRIME, Bost. Journ. Nat. Hist., Vol. VI., p. 363, pl. xii., 
fig. 11, 12, 1852. 
Pisidium notatum Prime, l.c., p. 365, pl. xii., fig. 20-22, 1852. 
Pisidium arcuatum PRIME, |. c., p. 364, pl. xii., fig. 14-16, 1852. 
Pisidium resartum INGALLS, in litt., 1855. 
Pisidium rubrum Lewis, in litt., 1855. 
Pisidium plenum LEwIs, in litt., 1855. 
Shell: Large (for the genus), inflated, rather solid, some- 
what inequilateral, elongate-oval; umbones slightly raised, 
full, placed a little posterior of the center, very rough, some- 
times showing growth lines plainly, but generally covered with 
