Transverse sections of cortical bone of the perissodactyl (a, b) and artiodactyl (c, d) samples viewed under transmitted light. The perissodactyl bone is characterized by a Haversian-type histology. Some cement lines are indicated with black arrows whereas a diagenetic crack is denoted by a white arrow in (a). Well preserved osteocyte lacunae are indicated with a black rectangular in (b). The artiodactyl bone has a characteristic fibro-lamellar structure with primary osteons. Remodelling in the periosteum area is evident in (c). Notation: SO, secondary osteons (Haversian systems); IL, interstitial lamellae; Res, resorption cavity; HC, Haversian canal; FLB, fibro-lamellar bone; PO, primary osteons; TC, transverse vascular canal.