Shell length up to 27 mm, mean w/1 ratio 0.79, mean h/1 ratio 0.39. Shell symmetrical, cap-like and relatively deep and wide. Apex outside the posterior shell margin and often eroded. Ground colour yellow-brown with a black or purple-pink pattern of triangles and horizontal lines. When the periostracum flakes off the shell, a purple pattern remains. Shell inside white to light gray, septum narrow and its edge curved and tinged yellow. Male shells significantly smaller than females and fewer males in the populations. Operculum: Pale pink, nearly square, robust with a relatively long rib. Horn margin yellow-orange.

S. porcellana has been confused with other Septaria species with a similar shell and this suggested that it had a wider distribution than it actually has. At one time its distribution was thought to extend from the Indian Ocean, across South East Asia, the Philippines, Indonesia to Taiwan and down across the Pacific to Fiji and French Polynesia.
RECLUZ (1841) erroneously combined it with the Indian Ocean species under the name of S. borbonica and later RIECH (1937) continued the practice. Various Septaria species have been mistakenly identified as S. porcellana - female S. suffreni by STARMUHLNER (1976), S bougainvillei by STARMUHLNER (1976), HAYNES (1985, 1988, 1990, 1991), S. taitana by STARMUHLNER (1976), POINTIER & MARQUET (1990), RESH et al. (1990), S. apiata by FOSSATI ET AL. (1992) and S.janelli by MACIOLEK & FORD (1987).
The holotype of S. porcellana, held in the Linnaean Society, London is a deep robust shell that has lost its periostracum and is, therefore, white with a pink pattern of triangle and horizontal lines. It is not much like the illustration of Patella porcellana in Rumphius, Georgius Everhaus, Amsterdam, 1705, which is cited by LINNAEUS. It has a few dark triangular lines. However, the P. porcellana holotype is like specimens collected from Solomon Islands, New Guinea, Vanuatu and New Caledonia which have a similar black-red pattern of triangles and lines. Specimens from Andaman Islands, India, Indonesia, Philippines, Taiwan and Northern Australia also conform to the holotype.
The two syntypes of Navicella depressa LESSON (MNHN) were collected from New Guinea. The larger of the syntypes is certainly S. porcellana although the smaller is less typical. As there is variation of shell pattern in all Septaria species, S. depressa should be considered a synonym of S. porcellana and not a subspecies.
Some S.janelli shells have a similar pattern to those of S. porcellana, but S.janelli shells are more green in colour, the apex is off centre and the males are the same size as the females.

New Guinea, Solomon Islands, Vanuatu, New Caledonia, India, Andaman Islands, Indonesia, Philippines, Taiwan, Okinawa, Guam, Saipan, Northern Australia. In still and swift current on stones from a few metres from the sea to 5 - 6 km inland.