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The smallest number of dorsal vertebrae known in reptiles is that of the Chelonia, invariably ten. In the chameleon lizards there are as few as eleven; in the pterodactyls about twelve. In the latter order three or more immovably united for the support of the pectoral arch, forming the notarium. In the Chelonia they are fused throughout in the carapace. The largest number of dorsal vertebrae in reptiles having a sacrum, forty-one or forty-two, is found in Pleurosaurus, a slender, aquatic Jurassic reptile. About thirty is the usual number in plesiosaurs. In terrestrial reptiles the number never exceeds twenty-two or twenty-three and is usually about eighteen. In reptiles lacking