known, in structure of the sexual reproductive areas, the family seems
to be closely related to the mainly non-lichen _Patellariaceae_ and to
such lichens as the _Gyalectaceae_, the _Lecanactidaceae_, the
_Collemaceae_, the _Baeomycetaceae_, and the _Cladoniaceae_.

Following the commonly-accepted theory that the lichens have been
evolved from non-algicolous fungi, the origin of the _Lecideaceae_ and
related lichens from _Patellaria_-like ancestors is a reasonable
supposition, though the relative rank of the various related families
named in the last paragraph is not easy to decide. Within the
_Lecideaceae_, the line of evolution seems to have been in the direction
of a well-developed exciple and from simpler to more complex spores.
With the advance in these two directions has gone a slightly increased
development of the thallus.

In structure, the thallus is crustose, and the thalli vary from
inconspicuous, evanescent conditions to those which are conspicuous and
sometimes even subsquamulous. Rarely the thallus extends upward as a
veil which surrounds the apothecia laterally and suggests how the
thalloid exciple of higher families probably arose. As usual in crustose
forms, the thalli are composed of hyphae which are densely disposed
toward the upper, exposed surface and more loosely disposed toward the
lower surface (Fig. 2).

The apothecial evolution passes from forms with weak, light-colored
exciples and soft texture (Fig. 10) to those with strong, dark exciples,
which are firm in texture (Fig. 11). The superficial apothecial
characters are so much alike in many of the species that one cannot
always feel certain even of the genus of unfamiliar forms until he has
studied them microscopically.