Ultrastructure of Meridion lineare D.M. Williams is documented from North American populations. This species differs from others in the genus by being only slightly asymmetrical about the transapical axis. The straight-chain filaments formed by interlocking spines in M. lineare may contain costate and acostate cells, the latter possibly being Innenschalen. Valve ultrastructure of M. lineare is compared with sympatric populations of M. circulare. Acostate valves of M. lineare are formed from costate valves and this ontogenetic sequence has possible implications for understanding character evolution and interrelationships of the freshwater, araphid diatoms. Innenschalen formation may be a feature that is synapomorphic for the genus Meridion.