The writer has been studying on the evolution of vertebrates by using shark teeth as materials Recent sharks have many specialized features, but they can be regarded as a relict form of the primitive gnathostomata vertebrates on account of their whole body structure. In the phylogeny of vertebrate hard tissues, it is considered that all sorts of basic tissue types were established in sharks and continue to mammals and men. Further, the genesis of the outer layer of shark teeth has been in controversy for more than one hundred years, one school regarded it as enamel, while the other called it a sort of dentin, and its true nature has not been made clear till today. In this paper, the writer described the development of shark teeth and discussed the evolution of teeth from a standpoint of phylogeny. The jaw teeth of living Triakis scyllia were used as materials and their developing process was observed by optical microscopy and scanning and transmission electron microscopy. The results are summerized as follows. 1) The order of tooth hard tissues formation is as follows: Formation of enamel matrix→formation of predentin and calcification of enamel→formation of dentin, formation of osseous tissue of root of the tooth and maturation of enamel. 2) The tooth germ is composed of the epithelial and the mesenchymal elements. The former consists of the inner and the outer enamel epithelia, which differentiated from the dental lamina. The mesenchymal element consists of the dental papilla and its surrounding tissues. The enamel organ which has the stellate reticulum as seen in mammals does not differentiate in the epithelial element of shark tooth germ. 3) The enamel matrix is formed on the dental papilla side of the basement membrane, and contains many micro-tubular structures, collagen fibrils which continue from Korff's fibers, and odontoblast processes. 4) Initially, the enamel crystallites are of needlelike shape, but later they become hexagonal rods which are similar to mammalian enamel crystallites. They do not develop uniformly as in mammals, but are irregular in size during development. 5) The odontblast becomes to contain well developed rough-surfaced endplasmic reticulums and Golgi apparatus concomitant with the formation of the enamel matrix, retreats centripetally extending many plotoplasmic processes, and differentiates into columnar cell. While, in the epithelial element, the inner enamel epithelial cell differentiates into columnar ameloblast, and at the same time the outer enamel epithelial cell differentiates as cuboidal cell. In the ameloblast, large aggregation of glycogen particles is observed at the matrix formation stage, and well developed Golgi apparatus and many granular structures are found at the calcification and maturation stages. The above results lead the writer to the following conclusion. It seems that the mesenchymal element contributes considerably to the enamel formation because of a low degree of differentiation in the epithelial element of shark tooth germ. This suggests that the tooth tissues of vertebrates, even enamel or dentin, are in general formed by the interaction between epithelium and mesenchym. Further, it is considered that the cause of tooth (enamel) evolution in vertebrates is directly controlled by the degree of differentiation potential of the tissues and cells of the tooth germ, but, fundamentally, it should be searched for in the evolutionary processes of whole body structure and mode of life.