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Thirty-four pups of the Northern elephant seal, Mirounga angustirostris, collected from Mexican and Californian coastal islands were studied to determine causes of death, growth of certain organs, and development of the supernumerary bones of the occiput. Dead pups from Añ Nuevo Island off central California from 1965 through 1969 appear to have died due to crushing by adults, probably the large breeding bulls. Recent deaths through the 1972 breeding season indicate that some deaths are attributable to bites inflicted by agitated cows. Organ weights were taken on several pups and compared with weights recorded for pups of the Southern elephant seal. Some organs appear to grow proportional to body weight, and other organs show disproportionate growth. The supernumerary bones of the occiput called “extraoccipitalia” were found to be embryonically preformed in cartilage. Ossification centres were shown to be developing in late term foetuses. These extraoccipitalia evidently allow rapid growth of the neurocranium during the first few months postpartum and later fuse with adjacent cranial bones at about one year of age. The rising elephant seal population in relation to the types of mortality is discussed in relation to available breeding grounds.