  CO J=1-0 mapping observations of the dark nebula Lynds 227, which is possibly
associated with the unidentified EGRET source 2EG J1811-2339, are presented. We
detected a large amount of molecular gas along Lynds 227 with a total mass of
(1-2)E4 Msun surrounding an X-ray synchrotron nebula, which was detected by the
ASCA satellite within the error circle of 2EG J1811-2339. Molecular gas along
Lynds 227 shows spatial anti-correlation with the X-ray synchrotron nebula,
suggesting that the synchrotron nebula is interacting with the Lynds 227 cloud.
We propose a $\gamma$-ray emission mechanism for 2EG J1811-2339: High energy
electrons are injected from a rotation-powered pulsar and accelerated further
in shock waves generated by the interaction with ambient matter. The high
energy electrons move into the molecular cloud at the Lynds 227 and collide
with dense interstellar matter to yield high energy $\gamma$-ray photons,
mainly through relativistic bremsstrahlung.
