Long bone histology of species of Nothosaurus . A-D, thin sections in diaphysial region; E, F, thin sections in metaphyseal region; A-C, E, in polarized light; D, F, in normal light. A, humerus of N. marchicus/N. winterswijkensi (MfN R 174–2) showing a cortex which consists of well vascularized LZB. There is a thin, highly birefringent circumferential layer of lamellar bone that separates the medullary region from cortical bone. Trabeculae of endosteal bone with a core of calcified cartilage partially fill in the medullary region; B, humerus of N. giganteus (SIPB R 40), showing Sharpey’s fibres in cortical LZB of the posterior sector of the cross section. Note how the fibers influence the appositional bone organization, creating a radial appearance of the cortical bone, and how the vascular canals are deflected radially. Detail of humerus of N. giganteus (SIPB R 53) in polarized (C) and normal light (D). C, the outermost cortex of this specimen is composed almost entirely of thin layers of lamellar bone; D, closely spaced LAGs (EFS) in the outermost cortex of a humeral cross section indicate that diaphyseal growth has nearly completely ceased. E, humerus of N. giganteus (MHI 1903), Section 2 (the relatively more distal one), has a cortex composed of RLLZB showing regularly spaced zones, annuli and LAGs. Arrows mark the visible LAGs. F, humerus of N. giganteus (MHI 1903), Section 1 (the relatively more proximal one), the highly vascularized lamellar zonal cortex merges towards the shaft into cyclial LZB as seen in E. Abbreviations: cc, calcified cartilage, eb, endosteal bone, emb, embryonic bone, sf, Sharpey’s fibers.