The purposes of this monograph are to describe the postcranial skeletons of the earliest known mammals, and to probe, in so far as possible by osteological study, biological questions concerning the habits and adaptations of these late Triassic forms. In this context, information on the background of this investigation is useful. Studies of Mesozoic mammals, begun some 150 years ago, are based on rare and fragmentary fossils, principally jaws and teeth. These investigations have yielded a bare outline of some 120 million years of mammalian evolution-about two-thirds of mammalian history. No assessment of the important biological changes occurring during this time can ever be complete, but major advances are possible as new discoveries provide material that is more complete or that represents a previously unknown evolutionary stage. So tenuous is the evidence that at least some concepts are re-evaluated with each discovery. Postcranial anatomy offers especially intriguing prospects for investigation because associated material (that can be positively assigned to a taxon below subclass) has been for the most part unknown, and indeed even dissociated bones are a rarity. Since G.G. Simpson's monographs of 1928 and 1929, progress in the study of Mesozoic mammals has been largely dependent on new finds. A major impetus to renewed investigation came from the discoveries of Mesozoic mammals by Walter Kühne in 1939 and during the immediate post-war years. Kühne first worked on fissures in the Carboniferous limestone quarries at Frome, Somerset, in southwest England where he collected a series of teeth of the problematical form Haramiya and two triconodont teeth which were placed in the genus Eozostrodon (Parrington 1941, 1946). The fissure faunas are generally thought to be of Upper Triassic (Rhaetic) age (Kühne 1946), although Kermack, Musset & Rigney (1973) believe that the evidence is insufficient to determine whether the deposits are Rhaetic or Lower Liassic. After the war Kühne carried his explorations farther west, eventually reaching the quarries at Bridgend in Glamorgan, Wales, where he not only found more triconodont teeth in some quantity (Kühne 1958) but also a symmetrodont tooth (Kühne 1950). Shortly after making these discoveries, Kühne returned to Germany and the work was continued by a team from University College, London, under the leadership of Dr K.A. Kermack.