In the cases of the Nares and Sohm abyssal plains we have taken the name Murray has given to the "deeps" in which the plains occur. For Cape Verde, Iberia, and Newfoundland abyssal plains we have followed the name Wüst (1940b) proposed for the basin within which the plains lie. The Biscay, Tagus, Hatteras, Blake-Bahama, Hispaniola-Caicos, and Balearic have been given the name of a prominent near-by land area, in accordance with recognized practice. The Horseshoe Abyssal Plain takes its name from the horseshoe-shaped line of seamounts which surrounds it except on the eastern side. * * * * * _Regional description._--The relief of the abyssal plains is so low that ordinary small-scale exaggerated profiles (Pls. 22, 27) reveal nothing of its detailed character. In a few areas exceptionally good PDR records have been obtained which show relief of a few feet (Pl. 12). SOHM ABYSSAL PLAIN: The best-studied abyssal plain is the Sohm Abyssal Plain south of Newfoundland. In addition to the 1953 VEMA PDR survey (Heezen _et al._, 1954) the area was studied extensively in 1955 and crossed by the trans-Atlantic cruises of 1954 and 1956. The plain is T-shaped and generally about 200 miles wide; depths range from 2700 to 3000 fathoms (uncorrected). The depth increases in each arm of the T toward the stem where the slope changes to south. The greatest depth is found at the south boundary of the plain near 29° N. In the northern part of the plain small topographic irregularities are unknown, but toward the south peaks 50 to 500 fathoms high increase in number until they finally replace the plain. North of the Bermuda Rise the plain surrounds a line of huge seamounts. However, even here no small hills are found. In the strip extending between 30° and 37° N., the east and west boundaries of the plain are formed by scarps 200 to 800 fathoms high. The boundary on the north side of the Bermuda Rise is formed by a distinct shallowing, but no scarp is seen. There is some justification for separating the northwest arm of the Sohm Abyssal Plain north of the Bermuda Rise from the rest of the Sohm Abyssal Plain since a mid-ocean canyon runs from the northwest to the central sections of the plain. A sill might lie northeast of the northeast tip of the Bermuda Rise. Here the bottom gradient changes slightly, and an extremely large magnetic anomaly is associated with the region. However, the plain narrows only slightly at this point, and, since we cannot tell how many individual basins were filled to form an abyssal plain, we will consider that the Sohm Plain now includes the whole area, keeping in mind that the northwest arm may have only recently merged with the rest.