The smallest, yet perhaps the most striking, Atlantic abyssal plains are those at the bottom of the Puerto Rico Trench at depths of about 4358 fathoms (4585 corrected). They exhibit the proper degree of flatness, low gradient (Ewing and Heezen, 1955), and the shallow-water clastic sediments typical of abyssal plains. South of Cuba in the southwest corner of the diagram lies a major deep-sea trench. Only the eastern end of this Cayman Trench is shown in the diagram. On the floor of this steep-walled trench lies a narrow trench plain or system of semi-connected trench plains which lie at depths of about 3000 fathoms south of Guantanamo, Cuba, but deepen to 3692 fathoms south of the Cayman Islands. These are known as the Cayman Trench Plains. This area will be described in more detail in a later publication devoted to the topography of the Caribbean. In the eastern Atlantic each abyssal plain has been crossed at least three times by ships employing a PDR, a coverage which, although sufficient to establish their existence, is vastly inferior to the coverage of the western Atlantic abyssal plains. The Biscay Abyssal Plain occupies a large portion of the Bay of Biscay. It is connected by an abyssal gap at 43° 30´N., 12° 00´W., to the Iberia Abyssal Plain which lies west of the northern two-thirds of the Iberian Peninsula. A small abyssal plain west of the Tagus River takes its name from this river. The great abyssal plain which skirts Madeira on the east and south and extends over a vast area to the west is referred to as the Madeira Abyssal Plain. The Canary Islands and associated banks form the boundaries of a small oval depression known as the Canary Abyssal Plain. The vast abyssal plain which skirts the African continent west of Cape Verde is named the Cape Verde Abyssal Plain. The eastern margin of the map just reaches the abyssal plain in the western Mediterranean which Heezen and Ewing (1955) have named the Balearic Abyssal Plain. Abyssal plains were not discovered until the use of continuously recording echo sounders was extended to abyssal depths following World War II (Tolstoy and Ewing, 1949). While the early nonprecision echo sounders were sometimes adequate to distinguish the limits of particular plains, real advances in their study required the development of precision sounders and the extensive use thereof (Heezen, Ewing, and Ericson, 1954). Since the very existence of abyssal plains has been known for less than 10 years and only recently have any of them been adequately delineated, the problem of their nomenclature has never before arisen.