Cymatium gibbosum reaches a markedly larger size than either C. labiosum or C. kobelti (largest C. gibbosum seen: Panama, ANSP36950, height 60.8 mm, diameter 37.2 mm). Of the 16 specimens measured, the greatest frequency of height falls equally in the 30-34.9 and 35-39.9 mm classes , but four of the specimens are larger than all C. kobelti examined. Most specimens from intertidal and shallow subtidal locations are short and wide, with an almost horizontal to slightly concave sutural ramp, a prominent, wide cord around the shoulder angle, and a short siphonal canal. Most specimens from deeper water are taller and narrower and have less prominently tabulate shoulders than shallow-water shells. This great range of shape is reflected in the histogram of h/d frequency by a wider range than in the other two species, from equal to the widest C. labiosum to equal to the tallest C. kobelti; the highest frequency is in the 1.80-1.89 class, higher even than that of C. kobelti. Of the 16 measured specimens, only two have h/d lower than 1.5. Almost all specimens have varices regularly situated at each 0.66 whorls down the whole teleoconch; varices are high and thin, slightly concave abaperturally, and almost flat-faced adaperturaJly. The teleoconch sculpture is dominated by spiral cords; few specimens have the regular, prominent collabral costae of C. labiosum. Teleoconch spiral cords begin on the first whorl as a group of three narrow riblets around the shoulder angle, another group of three narrow riblets on a weak angulation at the lower third of whorl height, a single interstitial thread in the centre of the relatively wide space between these two groups, and several low, narrow threads on the sutural ramp and below the lower primary cord. On both C. gibbosum and C. kobelti, the two initial trifid primary cords become more closely spaced over the one or two subsequent whorls, to develop into a wide, elevated zone of six riblets beneath the shoulder angulation, with the original cord interspace represented only by a weak median groove, while the remaining whorl surface below is clearly less elevated (i.e., the whorl has a smaller diameter) and more weakly sculptured. However, in both C. gibbosum and C. kobelti, the stage at which the relatively rapid transition occurs from the separated-cord form to the prominently keeled form is highly variable.
In some specimens the transition occurs on only the second half of the first teleoconch whorl whereas on other specimens it occurs on the second or third teleoconch whorl.The result is the same in all specimens of both C. gibbosum and C. kobelti: the last two or three whorls oflarge shells bear a massive, extremely prominent, wide cord at the shoulder angulation, crowded with narrow spiral threads, as in C. labiosum, and the separated trifid cords on the initial teleoconch whorl are reduced to weakly defined groups of three spiral threads over the remainder of the whorl surface (although the major cords remain weakly defined over the varices). The whole teleoconch surface bears regular, narrow, collabral costellae in both species, forming regular low gemmae at the intersections with the spiral threads. In both C. gibbosum and C. kobelti the aperture is muriciform and subcircular, with a thick, weakly reflected peristome, the interior of the outer lip weakly nodulous in conformity with the spiral cords and interspaces of the exterior surface. The teleoconch is uniformly coloured in most specimens, pale greyish brown to medium red-brown in both species. The protoconch of C. gibbosum is highly variable in size, tall and narrowly conical; its periostracum is pale yellow-brown and bears weak collabral riblets in the suture and, on little-abraded specimens, two narrow, very low, spiral threads bearing traces of former bristles. The spiral threads axe set low on the whorl, and in most specimens are present on the last whorl only.

Anchorage north of Cedros Village pier, Cedros Is., west coast of Baja California, 20 m (LACM 67-64, two shells) and Adair Bay, northernmost Gulf of California (numerous records) to off La Libertad, Ecuador, 2°12'30"S, in 6-10 m (LACM/AFH9-33), and to Negritos, northern Peru (Olsson, 1924:124); Galapagos Islands (Emerson and Old, 1965:118). Reported by Dall (1921:141) to extend as far north as San Pedro and San Diego, California, but there are none from north of Cedros Is., west coast of Baja California, in collections we have examined.