Shell small, very slender, whorls flat-sided. Sculpture with three strong spiral cords. Colour dark or very dark brown, never light, appearing uniform at first sight.
Protoconch of lighter colour, transparent, with little more than three whorls of spire, convex, and with an imprecise end. The suture is well marked and there is a small stria on it. The first whorl is small and, under magnification, it shows very small tubercles disposed in files. The second and the third whorls are a little bigger and do not show tubercles. At the end of these three whorls appears a cord in the convexity which is soon duplicated; at the same time, the colour turns brown, typical of the teleo¬conch, and axial striae appear, having the aspect of growth-lines. A little further, begins the third cord which is subsutural. There is not a clear beginning of the teleoconch. Half a whorl behind of the three and a quarter whorls of the protoconch, there are already three spiral cords. The teleoconch can attain 15 whorls of spire. The first ones show three spiral cords. After the sixth or seventh whorl there appears in the suture a new thinner cord that continues to the end of the shell, increasing slowly in size but being always a little smaller than the other ones and maintaining its suprasutural position. The space between the first and second cord is narrower than the one between the second and third cord. The uniform brown colour is present on the first 5-7 whorls, but on die last whorls appear light blotches aligned in an axial direction and which can be clearly observed on the cords. Within the spaces between the spiral cords, there are axial striae, which are vertical between the second and third cords and a little oblique above (between the first and second cord) and below (between the third and fourth) but in a different sense. Dimensions: The adult specimens studied reach between 9 and 12 mm. Most of the biggest specimens have lost their protoconch by erosion. The animal body is whitish, a little translucent, with a large quantity of milk white spots regularly distributed. Between the tentacles, there is an orange or light brown spotted area extending towards the dorsum and around the eyes and, sometimes, reaching the proximity of the operculum. This operculum is thick, corneus, of yellow colour, triangular in shape and curved. The radula has the typical aspect of the genus, with a central tooth with five pointed cuspids from which the central and the most external ones are the most prominent. The lateral tooth has around 10 very similar pointed cuspids. The marginal tooth is long and has its free extremity curved with three appendices: one of them is filiform, another one is pointed and curved over the third one, which is shorter and with a digitiform profile.

Seila angolensis n. sp. is different from S. adamsi which has a protoconch with less than three whorls, without tubercles on the first one and lacks the stria on the suture; the colour of the animal is different and the radula has a central tooth with very irregular cuspids. S. carinata has a slender shell and, with the same size or with the same number of whorls, its width is almost half that of S. angolensis. Moreover, the protoconch of S. carinata, though broken is wider and has more spiral whorls. S. trilincata shows spiral cords and very regular and similar spaces between them; its protoconch has only two spiral whorls. The differences with the other species that are described in the present work are shown in Table I and will be commented on the general discussion.

The type locality is designated the Corimba Bay in Luanda. (Angola). The present species has been collected on all the Angolan coasts. Habitat.
It was found in crevices in rocks and under rocks and bivalves, with small algae and sponges, dredged from 20 metres deep up to the intertidal zone.