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Webster 1828 Edition

Adj.

superl. of close. Most close. In these words, s has its proper sound.

closest

superlative form of close: most close.
- 1992, Rudolf M. Schuster, The Hepaticae and Anthocerotae of North America: East of the Hundredth Meridian, volume V, page 4
  The closest affinities of the Jubulaceae are with the Lejeuneaceae. The two families share in common: (a) elaters usually 1-spiral, trumpet-shaped and fixed to the capsule valves, distally […]