Writing claw
The writing claw can refer to the claw of the digit next to the inner thumb of the forepaw, or to the entire digit analogous to a human's index finger.
A yinrih's writing claw is shaped like the nib of a fountain pen, and acts more or less like one as well. However, while a fountain pen uses capillary action and gravity to cause a "controlled leak" to produce ink flow, a yinrih has voluntary control over the muscles in the ink sac. By contracting these muscles, and by gently pressing the tip of the claw against a writing surface, ink can flow out. This mechanism allows the writing claw to work in microgravity.
One of the few anatomical differences between yinrih and tree dwellers is the arrangement of the so-called "writing tract", the system including the writing claw, the ink duct, and the ink sac or bulb. In tree dwellers, the ink bulb and duct are located directly ventral to the phalanges, meaning each footfall applies pressure to the ink causing some to flow out passively onto the surrounding surface as the tree dweller walks or climbs. In yinrih, the ink duct and bulb have migrated slightly to the side, thus applying less pressure with each step, reducing or eliminating passive ink excretion. Presapient yinrih had to actively smear their ink onto surfaces in order to mark them, encouraging the development of the yinrih's primordial written language.
Yinrih have conscious control of the muscles that cause the ink bulb to contract, and ink flow relies more on the pressure applied by the contracting ink bulb than to gravity. This allows yinrih to write in zero-G.
Ink often dries on the tip of the writing claw, explaining the characteristic flicking gesture that yinrih do before writing.