1. On the surface of genital or anal lesions, either syphilitic or non-syphilitic, there may be found occasionally a spirochaeta resembling Treponema pallidum, but somewhat thicker than the latter. In general characteristics, this organism occupies an intermediary position between Treponema pallidum and Spirochæta refringens.
2. The treponema was obtained in pure culture from two cases. For the organism the name of Treponema calligyrum is proposed.
3. Treponema calligyrum is non-pathogenic for monkeys and rabbits.
4. Treponema calligyrum can be distinguished from Treponema pallidum, Treponema microdentium, Treponema macrodentium, Treponema mucosum, and Spirochæta refringens either by the morphology, cultural and biological properties, or all these conditions in combination.
5. Under certain cultural conditions Treponema calligyrum becomes thinner and may cause some difficulty of differentiation from a strain of the thick type of Treponema pallidum, especially when the latter is grown under conditions which tend to render it thicker. There is no such difficulty when the average and thin types of Treponema pallidum are concerned. On the other hand, by providing the calligyrum with conditions which lead to the appearance of thicker specimens and by supplying Spirochæta refringens with conditions of growth that favor the development of individuals that are thinner and more regularly curved, the refringens may be made to resemble the calligyrum in general appearance; but under identical cultural conditions they can be readily distinguished.