males, indicating sexual dimorphism. Dorsal Spine, Dentine, and Denticles The dorsal spines of Orthacanthus platypternus from the Craddock Bone Bed in Texas, USA, preserve a highly vascularized wall mainly composed of centrifugally growing dentine (the outer layer of the wall of the spine) in a succession of inwardly growing dentine layers that line the pulp cavity. These dentine layers are likely deposited periodically in accordance with seasonal variations in water temperature and food availability. More specifically, the periodic nature of the dentine layer deposits could be due to variation in calcium phosphate deposition following the changes in water temperature. Spines of individuals with 1-2 dentine layers are likely juveniles and result in the smallest sizes, whereas individuals showing at least 3-4 dentine layers result in two separate size classes. The cross section is oval near the opening of the pulp cavity and circular/subtriangular in the distal part of the non-denticulated region and circular in the denticulated region. The pulp cavity of the spine is filled with calcite, quartz, and opaque minerals. Occipital spine and denticles The spine is superficially inserted in the skin, where it grows and moves from a deep position in the dermis where trabecular dentine forms, to a superficial location where centrifugally growing lamellar dentine forms. The number of denticles per annual cycle vary with growth rate, and are independent dermal elements formed by the dermal papilla and secondarily attached by dentine to the spine proper. The density of denticulation also varies with the growth rate of the occipital spine. The ratio of length of denticulated region to total length of the spine changes throughout ontogeny. Historical information and discovery Genera Orthacanthus and Pleuracanthus were founded by Agassiz in 1837 on isolated "ichthyodorulites" from the British Carboniferous System, and at the time were mistakenly thought of as the first indicators of Skates. They were initially found throughout the United Kingdom in Dudley, Leeds, North Wales, Carluke, and Edinburgh. Three additional species from the Carboniferous formation of Ohio were described by Dr. Newberry. Teeth associated with Diplodus, a genus of sharks, was found in the Carboniferous slates of England in Stafford, Carluke, and Burdiehouse, and in Nova Scotia. A well preserved impression from Ruppelsdorf, Bohemia was described by Goldfuss, while a separate paper published the same specimen under the name Xenacanthus Dechenii. One year later in 1849, Dr. Jordan mistakenly identified the same specimen as the remains of