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The word is derived from the Fr. coutelas, or coutelace, a form of coutel, modern couteau, a knife, from Lat. cultellus, diminutive of culter, a ploughshare, or cutting instrument.

From topotypes of T. b. cultellus, T. b. rubidus differs as follows: paler; larger in all measurements taken; rostrum proportionately wider; zygomatic arches more rounded, less angular; angle formed by zygomatic arch and rostrum greater; bullae proportionately smaller, not so pointed anteriorly; alveolar length of upper maxillary tooth-row shorter.

T. b. pervagus occupies an area geographically intermediate between T. b. aureus to the west and T. b. internatus and T. b. cultellus to the east and has some characters in common with these subspecies.

Nevertheless, the similarities between T. b. pervagus and T. b. cultellus and T. b. internatus suggest that T. b. pervagus was originally derived from the more eastern stock.

T. b. internatus and T. b. cultellus probably intergrade east of the Sangre De Cristo Range in the vicinity of the Colorado-New Mexico boundary.