The shell is of moderate size for the genus (maximum length 22 mm) and roughly fusiform. The spire is high and acute, consisting of one and three-fourths nuclear whorls and seven weakly subangulatc postnuclear whorls. The suture is obscured. The body whorl is of moderate size and narrowly trigonal. The aperture is small and subovate, the posterior margin flattened. The peristome is barely erect, except parietally, where it is adherent. Each short, stout anal siphonal tube originates at the shoulder margin as an integral part of a varical shoulder spine, which originates immediately after the preceding varix and sweeps ventrally and posteriorly. The siphonal canal is of moderate length, comparatively narrow, sealed except for its distal one-third, and weakly, dorsally recurved.
The body whorl bears three weakly developed varices, each with a coarsely serrate margin and a single broad-based, dorsally and posteriorly projecting spine, this housing the anal tube. A series of amorphous, confluent, obliquely aligned nodes extends from the base of the varical spine of the previous varix to a point about medial on the current varix. Spiral sculpture, ephemeral except at the varical margin, consists of seven weak cords on the canal and three bifid cords on the body and on the shouldermargin spine. Shell color is dark red-brown to dark purple-brown intervarically, with pale red-brown on the varices; the leading edge of the last varix, the siphonal canal, and the peristome are white. The interior of the aperture is white mottled with red-brown. A thin, white frosting of intritacalx covers the entire shell.

Type locality. Mazatlan, Sinaloa, Mexico, 23° 1 TOO" N, 106°26'00" W, on rocky reef.
Distribution. Mazatlan, Sinoloa, Mexico, to Bahia Banderas, Jalisco, Mexico (DuShane, 1969; Hendrickx et al, 2005; Brusca & Hendrickx, 2008; Houart & Hendrickx, 2020), intertidal (Fig. 1).

Description. Shell length to 21.6 mm, width to 10.4 mm (Draper, 1980), medium sized for genus, rhomboid shaped, sharply triangulate, spire tall, acute, subsutural ramp tabulate, functional tubes short, outward projecting, pitting indistinct in holotype, except in varices where it is strong. Spiral cords strong, projecting, varices moderately strong, color off white to light yellow with several brown lines, one below the shoulder, an additional line at the base; intritacalx absent in holotype. Last teleoconch whorl spiral cord morphology with P1, s1, t, P2, P3, P4, s4, P5, s5, P6, ADP and MP. Aperture ovate, columellar wall thick, wide near axis, heavily calloused near posterior end, slightly projecting throughout, except near parietal wall, with Dl strong, D2 and D3 subtly split, D4-D6 weak. Siphonal canal comparatively narrow, short, 21% of total shell length in holotype, ventrally sealed, dorsally bent at tip.

Discussion. The geographical distribution is known from the original description and verified by others (see Distribution section above) which ranges from Mazatlan, Sinaloa to Bahia Banderas, Jalisco, Mexico. Tripterotyphis arcana is different than other eastern Pacific species in lacking an IP cord, multiple split PI cords, but having one tertiary cord after si and six apertural denticles. Although T fayae also has six denticles, that species has s2, s3, s6, ads, ms, ABP and three additional tertiary cords, all characters not seen in T arcana.

Known to us only from Mazatlan, Mexico (type locality), and La Cruz, Nayarit, Mexico.