Their names were Benjamin Jones and Alexander Carson.

They had been for two years past hunting and trapping near the head of the Missouri, and were thus floating for thousands of miles in a cockle-shell, down a turbulent stream, through regions infested by savage tribes, yet apparently as easy and unconcerned as if navigating securely in the midst of civilization.
The acquisition of two such hardy, experienced, and dauntless hunters was peculiarly desirable at the present moment.

They needed but little persuasion.

The wilderness is the home of the trapper; like the sailor, he cares but little to which point of the compass he steers; and Jones and Carson readily abandoned their voyage to St.Louis, and turned their faces towards the Rocky Mountains and the Pacific.
The two naturalists, Mr.Bradbury and Mr.Nuttall, who had joined the expedition at St.Louis, still accompanied it, and pursued their researches on all occasions.

Mr.Nuttall seems to have been exclusively devoted to his scientific pursuits.