sometimes referred to as an elodent dentition. These teeth grow or erupt continuously. The growth or eruption is held in balance by dental abrasion from chewing a diet high in fiber. Rodents Rodents have upper and lower hypselodont incisors that can continuously grow enamel throughout its life without having properly formed roots. These teeth are also known as aradicular teeth, and unlike humans whose ameloblasts die after tooth development, rodents continually produce enamel, they must wear down their teeth by gnawing on various materials. Enamel and dentin are produced by the enamel organ, and growth is dependent on the presence of stem cells, cellular amplification, and cellular maturation structures in the odontogenic region. Rodent incisors are used for cutting wood, biting through the skin of fruit, or for defense. This allows for the rate of wear and tooth growth to be at equilibrium. The microstructure of rodent incisor enamel has shown to be useful in studying the phylogeny and systematics of rodents because of its independent evolution from the other dental traits. The enamel on rodent incisors are composed of two layers: the inner portio interna (PI) with Hunter-Schreger bands (HSB) and an outer portio externa (PE) with radial enamel (RE). It usually involves the differential regulation of the epithelial stem cell niche in the tooth of two rodent species, such as guinea pigs. The teeth have enamel on the outside and exposed dentin on the inside, so they self-sharpen during gnawing. On the other hand, continually growing molars are found in some rodent species, such as the sibling vole and the guinea pig. There is variation in the dentition of the rodents, but generally, rodents lack canines and premolars, and have a space between their incisors and molars, called the diastema region. Manatee Manatees are polyphyodont with mandibular molars developing separately from the jaw and are encased in a bony shell separated by soft tissue. Walrus Walrus tusks are canine teeth that grow continuously throughout life. Fish Fish, such as sharks, may go through many teeth in their lifetime. The replacement of multiple teeth is known as polyphyodontia. A class of prehistoric shark are called cladodonts for their strange forked teeth. Unlike the continuous shedding of functional teeth seen in modern sharks, the majority of stem chondrichthyan lineages retained all tooth generations developed throughout the life of the animal. This replacement mechanism is exemplified by the tooth whorl-based dentitions