Adams described M. modesta as "reddish black...with, on the upper whorls, two, and in the middle and lower whorls three spiral ridges, which are of equal size on the lower two-thirds of the shell." "...whorls about fourteen..." Rolán adds in his 1995 paper, "Protoconch...with about 4 whorls, nucleus with minute hemispheric granules, subsequent whorls with two spiral ridges crossed by numerous axial riblets." Redfern further describes it as "To 5mm, sinistral, with a protoconch of 4.5 whorls..." "The adult shell tends to be relatively narrow and straight-sided, with the sutures well defined." He notes "A thin spiral thread is sometimes quite prominent at the suture." As I've observed, Redfern also notes that the beads are most often paler, especially against the darker background band of the suture and first subsutural cord.

Redfern comments that "This (M. modesta) is probably the species that has often been referred to as Triphora nigrocincta (C. B. Adams, 1839)." Most workers now treat the shell found in Florida that was previously identified as M. nigrocincta (C. B. Adams, 1839)( type locality - Dartmouth Harbor, Massachusetts) as M. modesta. 

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However, in 2021 Fernandes, et al., published the first  molecular study in Triphoridae examining the problematic complex “Marshallora nigrocincta,” which included M. modesta.  Their study established that both species occurred in Florida and provided a basis to distinguish the two.  This presentation has been updated to reflect the study findings.