Heterostraci ("Different scales") is an extinct class of jawless vertebrate that lived primarily in marine and estuary environments. They arose during the Ordovician, and all, save for the Psammosteids, became extinct during the late Devonian. This last group of heterostracans died out in the extinction event at the end of the Devonian. Description and anatomy The Heterostracans differed from other Paleozoic agnathan classes both in the arrangement of their scales, as well as the histology of their scales. Most heterostracans had two plates which form a large dorsal shield and a large ventral shield, and had series of scales arranged in various patterns on the sides of their bodies, the exact pattern differing from one group to another. In a few primitive forms, such as Lepidaspis, the dorsal and ventral shields are composed of a mosaic of tiny scales. In most other known forms, though, these tiny scales have fused together to form the shield-plates. The scales of heterostracans are histologically distinct from other vertebrates, having three layers composed of dentine and aspidine, an acellular bony tissue unique to this class. The middle layer was honeycombed with tiny spaces called "cancella." As with many agnathan groups, heterostracans had no fins besides the tail or caudal fin. In some pteraspids, especially in the psammosteids, the ends of the branchial plates (the plates that covered the gills) is drawn out to form wing-like extensions. [edit]Taxonomy Heterostraca is divided into two main groups, the orders Cyathaspidiformes ("Cup Shields"), and Pteraspidiformes ("Wing Shields"). The Silurian heterostracan Athenaegis is regarded as being a sister group of both orders, while the Early Devonian Lepidaspis is regarded as being incertae sedis, possibly close to the original basal forms. Cyathaspidiformes is divided into two main groups: the Amphiaspida of Early Devonian Siberia (grouped together with their relative Ctenaspis of Canada), and the Cyathaspidida (grouped together with their relati

e Nahanbiaspis. The Pteraspidiformes is divided up into five families, Anchipteraspididae, Protopteraspididae, Pteraspididae, Protaspididae, and Psammosteidae. The Psammosteidae are a family of flattened, benthic jawless vertebrates that lived in marine and estuary environments in Europe, Russia & North America. They arose during the Early Devonian, with the first (and best) known genus, Drepanaspis from the Hunsruck lagerstatte. The Psammosteids were the only heterostracans that survived the Upper Frasnian extinction event during the Late Devonian, dying out in the extinction event at the very end of the Devonian. Many of the Late Devonian genera, such as Psammolepis, were among the largest heterostracans ever, growing to be at least 1.5 metres in width. Dentine (British English: dentine, American English: dentin, Latin: substantia eburnea) is a calcified tissue of the body, and along with enamel, cementum, and pulp is one of the four major components of teeth. Usually, it is covered by enamel on the crown and cementum on the root and surrounds the entire pulp. By weight, seventy percent of dentin consists of the mineral hydroxylapatite, twenty percent is organic material and ten percent is water.[1] Yellow in appearance, it greatly affects the color of a tooth due to the translucency of enamel. Dentin, which is less mineralized and less brittle than enamel, is necessary for the support of enamel.[2] Dentin consists of microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp to the exterior cementum or enamel border.[3] These tubules contain fluid and cellular structures. As a result, dentin has a degree of permeability which can increase the sensation of pain and the rate of tooth decay. The formation of dentin, known as dentinogenesis, begins prior to the formation of enamel and is initiated by the odontoblasts of the pulp. Unlike enamel, dentin continues to form throughout life and can be initiated in response to stimuli, such as tooth decay or attrition.