The following radiograph shows; the total number of teeth present in oral cavity are one less than normal:
Fusion
Fused teeth arise through union of two normally separated tooth germs. Depending upon the stage of development of the teeth at the time of the union, fusion may be either complete or incomplete. It has been thought that some physical force or pressure produces contact of the developing  teeth  and their subsequent fusion. If this contact occurs early, at least before calcification begins, the two teeth may be completely united to form a single large tooth. If the contact of teeth occurs later, when a portion of the tooth crown has completed its formation, there may be union of the roots only. The  dentin; however, is always confluent in cases of true fusion. The tooth may have separate or fused root canals, and the condition is common in the deciduous as well as in permanent dentition.
Radiological signs
The tooth has a notched incisal margin and there is evidence of conjoined dentine between the involved teeth. The fused tooth may present with two distinct pulp chambers/pulp canal systems or a joined system.
Fusion
Gemination
Concrescence
Dilaceration