Developing cartridge including lock rib positioned at end surface of casing

A developing cartridge comprising:

• a casing configured to accommodate developing agent;
• a developing roller rotatable about a first axis extending in a first direction, the developing roller being positioned at one end portion of the casing in a second direction crossing the first direction;
• a coupling positioned at one end of the casing in the first direction and rotatable about a second axis extending in the first direction;
• a coupling gear rotatable about the second axis extending in the first direction together with the coupling;
• a detection gear positioned at another end of the casing in the first direction, the detection gear being rotatable about a third axis extending in the first direction;
• an agitator configured to agitate developing agent in the casing and configured to perform power transmission from the coupling gear to the detection gear;
• a first gear cover covering at least the coupling gear;
• a second gear cover covering at least a portion of the detection gear;
• and
• a lock rib positioned at the another end of the casing in the first direction and positioned opposite to the developing roller with respect to the detection gear in the second direction, the lock rib being configured to be locked to a portion of a drum cartridge in a state where the developing cartridge is attached to the drum cartridge.