Polygnathus costatus partitus is the standard fossil at the base of the MiddleDevonian. But it is difficult to distinguish it in morphology from Polygnathuslingui formis bultyncki occurring together with Polygnathus costatus partitus,which makes it difficult to define the Lower-Middle Devonian boundary. Detailedstudy shows that two developmental stages (young stage and adult stage) ofpartitus may be distinguished on the basis of the position of the cavity. Theontogenetic development of bultyncki involves three stages (larval stage, youngstage and adult stage) on the basis of its morphology. Comparison of the develo-pmental stages of the two subspecies reveals three differences between partitusand bultyncki, 1. Partitus's carina reaches the posterior end during its very early stage andretains this position through life; but bultyncki's carina oversteps the posteriorend of the platform during its larval stage, and terminates before the tongue duringthe young stage and adult stage. 2. The inner and outer posterior margins of partitus are characteristicallystraight, converging, to form a sagittat outline;whereas bultyncki develops a tongue. 3. The cavity of partitus of young stage oversteps the anterior end of the pla-tform and is located near the anterior end of the platform during its adult stage;but the cavity of bultyncki is located near the anterior end of the platform inthe larval stage and at one third length of the platform in the young stage andadult stage. These results supply the evidence for the fossil identification.