LYMN-EIDZ OF NORTH AMERICA. 197, 
Remarks: Megasoma is our finest American Lymnzea, not even 
rivalled by stagnalis. It seems fairly common in its range, which is 
decidedly northern. There is considerable variation in the height of the 
spire, some shells having a rather high, broadly conic spire, while 
others have a short, dome-shaped spire. These differences, as well as 
certain variation in color, are very trivial and are but individual modi- 
fications. This species cannot be mistaken for any other. A number 
of interesting experiments have been performed with these animals, 
mention of which has been made in Chapter II (see p. 47). In the 
spring of 1868 Dr. Whitfield introduced living megasoma into several 
ponds and streams about Albany. In 1871 he records the finding of 
one specimen in a pond. As no subsequent records have been seen, it 
seems probable that the various colonies died out, possibly because of 
unfavorable environment. 
Genus ACELLA Haldeman. 1841. 
1841. Acella Hatp—EmMAN, Mon. Limn., part 3, p. 6, July, 1841. Type, Lymnea 
gracilis Jay. 
1865. Acella BINNEY, L. and F. W. Sh. N. A., II, p. 69. 
1870. Acella Dati, Ann. Lyc. Nat. Hist. N. Y., IX, p. 349. 
1872. Acella Tryon, Con. Hald. Mon. Limn., p. 87 (61). 
1876. Acella MEEK, Rep. U. S. Geol. Sur. Terr., IX, p. 533. 
1884. Acella Tryon, S. and S. Conch., III, p. 101. 
1905. Acella Dati, Alaska Moll., p. 64. 
1908. Acella BaKxer, Science, N. S., XXVII, p. 943. 
SHELL: Thin, acute, with slender spire; outer lip slightly ex- 
panded at the margin, simple; axis gyrate ; columella not plicate ; sculp- 
ture of growth lines only (pl. XVIII, fig. 1). 
Jaw: Very high, with a strong, prominent median swelling. 
RapuLtaA: With bicuspid lateral teeth, the mesocones of which are 
strongly modified by a large swelling on the inner side, corresponding 
in position to an entocone. Intermediate teeth strongly tricuspid (pl. 
aie ne: BF). 
GENITALIA: Penis-sac long, cylindrical, the penis thick, 4/6 to 
4/7 the length of penis-sac; prostate large, flatly-cylindrical (pl. XI, 
fig. F). 
DiIsTRIBUTION: Nearctic; recent fauna only. 
Acella differs generically from all other Lymnezidz in its shell, 
all the whorls of which are very long in proportion to their width, even 
the nuclear whorls, a condition not shared by any other American 
Lymneas (see pl. XLIX, fig. E). The prostate differs in being long 
and regularly cylindrical. The radula is the most peculiar of any 
Lymneid yet examined, the mesocone of the lateral teeth having a 
