LYMN.EIDiE OF NORTH AMERICA. 191 



Remarks : Megasoma is our finest American Lymnsea, 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 Haldeman, Mon. Limn., part 3, p. 6, July, 1841. Type, Lymncea 



gracilis Jay. 



•1865. Acella Binney, L. and F. W. Sh. N. A., II, p. 69. 



1870. Acella Dall, Ann. Lye. 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., Ill, p. 101. 



1905. Acella Dall, Alaska Moll., p. 64. 



1908. Acella Baker, 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 (pi. XVIII, fig. 1). 



Jaw : Very high, with a strong, prominent median swelling. 



Radula : 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 (pi. 

 VIII, fig. E). 



Genitalia: Penis-sac long, cylindrical, the penis thick, 4/6 to 

 4/7 the length of penis-sac; prostate large, flatly-cylindrical (pi. XI, 

 fig. F). 



Distribution : Nearctic ; recent fauna only. 



Acella differs generically from all other Lymnseidse 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 

 Lymnseas (see pi. XLIX, fig. E). The prostate differs in being long 

 and regularly cylindrical. The radula is the most peculiar of any 

 Lymnceid yet examined, the mesocone of the lateral teeth having a 



