Abstract: An ink pen cartridge supported from a mounting arm for use on a recording instrument in which the attachment between the cartridge and mounting arm provides a secure and positive connection which accurately positions the ink cartridge thereby avoiding recalibration of the recorder when the ink in the cartridge is depleted necessitating a replacement with a filled cartridge. The exterior of the cartridge body is free of external arm connections and the arm is connected to the end of the cartridge to avoid interference between passing cartridges when mounted to multiple pen recorders. In addition, the ink cartridge has multiple connections in one end for optional attachment to a mounting arm whereby adjacent pens in multiple pen recorders may be more advantageously spaced, relative each to the other. The ink cartridge normally sealingly holds ink in the body, but the attachment of a mounting arm to the cartridge opens an air vent to the cartridge body for allowing flow of ink from the body.