Holotype: Shell small for the genus, surface dull white; aperture 1/3 the length of the shell, suture impressed, undulating, subsutural area markedly concave, periphery strongly shouldered. Protoconch of 3,5 smooth whorls, tip small and immersed; postnuclear whorls 7,5. Axial ribs 6 on early whorls, increasing only to 6 ½ on mature whorls, lacking in the subsutural area, strong at the shoulder and across the body whorl; back of last whorl with a more prominent axial rib 1/4 turn back; spiral sculpture of evenly spaced spiral incisions, faint on early whorls, more prominent on penultimate and final whorl, deeply incised across the pillar. Sinus deep, U-shaped, strongly outward projecting, bordered on the inside by curved parietal callus; lip edge thin, stromboid notch moderately deep; anterior canal scarcely defined, deeply notched; inner lip strongly raised above the pillar. Height, 11.3, diameter, 4.4 mm.

Some specimens of Drillia tumida show a faint pink tinge but most are uniformly white. It has the general proportions of D. roseola (Hertlein Strong, 1955), from which it differs in its much smaller size, presence of spiral sculpture, fewer axial ribs and less pronounced pink coloration. The Latin name means swollen, with reference to the massive axial ribs.

Pacific Central America. Type Locality: Banderas Bay, Jalisco, Mexico (approximately 20°43'N, 105°25'W), 20-40 fathoms