Tedford, R. H. (Department of Vertebrate Paleontology, The American Museum of Natural History, New York, New York 10024) 1976. Relationship of pinnipeds to other carnivores (Mammalia). Syst. Zool. 25:363–374.—Classical studies in comparative anatomy in the last century led to the recognition of a tripartite subdivision of the living Carnivora, modified in this century into a bipartite subdivision formalized in most classifications at superfamily rank (Canoidea and Feloidea). Osteological characters, particularly those of the cranium, support the sister-group relationship of the Canoidea and Feloidea and permit their recognition into early Oligocene time. Analysis of the Canoidea further supports the sister-group relationship of the Canidae (Cynoidea) with the Ursidae-Procyonidae-Mustelidae (Arctoidea) as classically perceived. These groups are also recognizable on osteological criteria in the Oligocene. Recent studies in the comparative anatomy, paleontology and immunology of the pinnipeds have reinforced their phylogenetic affinity with the Arctoidea. Paleontology and comparative anatomy in particular have presented a convincing case of sister-group relationship of the Ursidae (especially the Amphicyno-dontinae) with the Otarioidea (including the extinct Enaliarctinae). A major gap in the early history of the Phocidae limits paleontological contribution to the question of their precise sister-group within the Arctoidea. The early Miocene aquatic arctoid Potamo-therium fulfills many of the morphological criteria deduced for a phocid sister-group when autapomorphous characters of the living and fossil phocids are removed. Postcranial resemblances are particularly strong reflecting the reliance of Potamotherium on body flexion in swimming as in the phocids. Potamotherium retains a differentiated dentition and cranial characters which can be compared with other arctoids and these agree most unequivocably with those of the mustelids suggesting a specific sister-group relationship of the Mustelidae with the seals and Potamotherium. Thus the osteological evidence supports a specific sister-group relationship of the otariids with the ursids and of the phocids (including Potamotherium and its close relative Semantor) with the mustelids within the Arctoidea. By contrast immunological comparisons provided to date suggest a sister-group relationship of the pinnipeds as a whole with the remaining Arctoidea. A cladogram (Fig. 5) summarizes the criteria for recognition of the sister-groups within the Canoidea and suggests a new classification which would express the phylogenetic conclusions of this paper.