Deciduous teeth first teeth seen in the mouth are the mandibular centrals and the last are the maxillary second molars. The primary teeth are made up of central incisors, lateral incisors, canines, first molars, and secondary molars; there is one in each quadrant, making a total of four of each tooth. All of these are gradually replaced with a permanent counterpart except for the primary first and second molars; they are replaced by premolars. The replacement of primary teeth begins around age six, when the permanent teeth start to appear in the mouth, resulting in mixed dentition. The erupting permanent teeth cause
Hinge teeth sides). This arrangement is characteristic of the venus clams (Veneridae), the cockles (Cardiidae) and several other important groups. In the "desmodont" hinge, also known as an "asthenodont" hinge, the hinge consists of a large ligamentous resilifer (or chondrophore) which replaces the cardinal teeth, as in the soft-shell clams (Myidae). The "anodont" hinge is characterized by a strong ligament (or a series of transverse ligamental grooves as in the "Isognomonidae"), however, true teeth are absent in adults as is the case in the pen shells (Pinnidae), tree oysters (Isognomonidae), the pearl oysters (Pteriidae), and some freshwater mussels such as the genus