Marine mammals and sea turtles, particularly the leatherback, offer a unique opportunity to study the basic mechanics of endochondral and membranous bone formation unlike terrestrial mammals. These animals do not undergo the formation of a medullary cavity. Furthermore, there is minimal remodeling at a gross trabecular level such that the two patterns of bone formation - endochondral and membranous - remain side by side throughout much of skeletal maturation. Specific aspects of this, the modification of the patterns in some of the more unusual cetaceans, such as the humpback, and the rather unique almost mammalian-like characteristics of the leatherback turtle will be discussed. Also to be discussed will be the formation of secondary ossification centers in the carpal bone of the large cetaceans, a phenomenon not previously reported in other mammals. Reasonably concise models can probably be formulated from and understanding of these patterns of development.