Users can benefit from using an auxiliary peripheral that could mitigate many concerns with direct text-entry on wearable devices. I introduce ThumbText, a thumb-operated text-entry approach for a ring-sized touch surface. Through a multi-part exploration, I first identify a suitable discretization of the miniature touch surface for thumb input. I then design a number of two-step selection techniques for supporting the input of at least 28 characters. On a miniature touch surface, I find that a continuous touch-slide-lift selection technique in a 2×3 grid discretization offers improved performance gains over other selection methods. Finally, I evaluate ThumbText against techniques also designed for wearable devices and find that ThumbText (11.41 words-per-minute) allows for higher text-entry rates than SwipeBoard (6.49 words-per-minute) and H4-Writer (6.83 words-per-minute). I finally demonstrate that with ThumbText, users can benefit from a unique text-entry technique that transfers well across different wearable displays, such as smartwatches and head-worn displays.