Shell small, very slender, whorls flat-sided, with three strong cords, appearing dark brown at first sight. Protoconch with little more than two whorls, relatively broad (0,4 mm), with its first whorl covered with small tubercles, and its second one smooth or with smaller and more isolated tubercles. Teleoconch dark brown but showing under magnification small light blotches on the spiral cords. The whorls are from nine to eleven, increasing slowly in size; the suture is inconspicuous. Three smooth spiral cords run all over the shell, the lowest of them being a little more prominent. On the fourth or fifth whorl appears a fourth cord on the suture, that never becomes as big as the preceding ones; on the last whorl are five spiral cords. In the interspaces, axial flakes are visible. The aperture is rectangular with the external edge sharp and undulating at the ends of the spiral cords. The siphonal canal is open and short. The specimens studied have their maximum dimensions between 6 and 8 mm.

Seila parilis n. sp. is different from S. trilineata because the latter species has a more uniform colour, its protoconch has a small stria on the suture and there are two cords at the beginning of its teleoconch. S. carinata is more slender and elongated and it has a protoconch with more whorls; the spiral cords of the teleoconch and the interspaces are different S. adamsi lacks the tubercles on the protoconch and it is also smaller; the central tooth of the radula is very different, with more pointed cuspids. S. angolensis has a protoconch with more whorls, a smaller nucleus and has a single cord (instead of the three simultaneously) at the beginning of the teleoconch.

Type locality. Esprainha, Sao Tome Island.
Only known from Sao Tome Island. S. parilis n. sp. was found on stones (by brushing) from 2 to 4 m depth.