1889.1 On Variation. 
1075 
that the most experienced are often at fault and perplexed with 
the difficulty of placing them properly in the most approved sys- 
tem." He further remarks upon the variableness of the features 
upon which the species of Unio are characterized. For example, 
the teeth vary in the same species from one angle to another; 
they are thick in one specimen and thin in another ; corrugated 
m some and in some smooth. The color varies in the same spe- 
cies, both in the nacre and in the epidermis. In most specimens 
of Unio gibbosus the nacre is dark purple, but it is also sometimes 
white. In Utiio verrucosus it is generally dark chocolate, but it 
also varies to white. Mr. Lea says he has an Anodonta from the 
Ohio with the nacre of one valve white and the other salmon 
color. In certain species of Unio claims the epidermis is beauti- 
fully rayed, but other specimens have no rays at all. In Unio 
cesopus the epidermis is sometimes glossy yellow, and sometimes 
dark brown. This is ako the case with Unio cylindriciis. Unio 
alatus again varies from a beautifully rayed green to nearly black 
and rayless. The tuberculations and undulations vary. Speci- 
mens of Unio lacrymosiis, normally with numerous tubercles, are 
sometimes nearly smooth. Unio plicatus may have a few or 
have numerous folds, or even be nearly smooth. "The Unio cor- 
mitns is furnished with three or four protuberances or horns in a 
row, passing from the backs direct to the basal margin ; the vari- 
eties of cornntus have these ' horns ' more depressed and more 
frequent, and thus pass into varieties with a mere furrow without 
any distinct elevation, and these gradations are almost innumera- 
ble." The beak varies in the same species, as does also the 
general outline of the shell ; as, for example, in Unio Intcolus, 
■which varies from oblong "pea-shaped" to a shorter form, with 
a broad anterior basal projection. The muscular impressions on 
the interior vary, as does also the ligament. In short, there is 
no character so constant that it can be made the certain charac- 
teristic of any one species. (See Ibid, pp. 407-415-) 
Still later (in 1870) Mr. Lea returns to the subject of variation, 
and again calls attention to the fading of one species into another, 
and the difficulty of drawing lines of separation with any definite- 
ne.ss. (Synopsis of Unionidae, p. 1 1 .) 
