A container assembly such as a petri dish or a contact plate for use as a sampling device for microorganisms includes a base member, a lid and a locking mechanism that provides a secure locking engagement between the lid and the base member. The locking mechanism is designed such that does not lock except upon application of a specific intentionally applied compressive force, and which may be readily disengaged from locking engagement without the need for a rotational movement or torsional force.