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Six species of large-celled PinnulariaEhrenberg from lakes of eastern North America, P. dariana(A.W.F. Schmidt) Mills, P. alabamaeKrammer, P. bihastata(A. Mann) R.M. Patrick, P. torta(A. Mann) R.M. Patrick, P. rexloweisp. nov. and P. spiniferasp. nov. are investigated with light and scanning electron microscopy. All six species are shown to possess marginal spines, a trait sporadically encountered in Pinnulariaand generally rare in raphid diatoms. Besides having spines, the six species differ in their frustular morphology, including appearance and position of spines. We summarize data on their ecology and distribution using a set of 702 sediment core samples from 424 lakes from eastern USA and Canada. Pinnularia rexloweiis only known from Quaternary lacustrine deposits in the State of Connecticut and from the surface and subfossil sediments of two lakes in northeastern USA. The other five species are relatively common, and at least one species of spine-bearing Pinnulariawas found in 34% of studied lakes. Pinnularia darianaand P. tortareach as far north as Labrador, while P. alabamae, P. bihastataand P. spiniferawere only encountered south of Canada. Pinnularia alabamaeand P. spiniferaare characteristic for lakes with relatively soft and slightly acidic water, while P. bihastata, P. darianaand P. tortaare found in a wider range of physico-chemical conditions, including alkaline and eutrophic lakes.

Cyclotella lacuskarlukiManguin ex Kociolek & Reviers is a small diatom known only from the original drawing that provides little information on the morphology of this taxon. As Manguin’s original materials are lost, recently collected samples from Lake Karluk, Alaska, the type locality of the species, were studied with light and scanning electron microscopy to determine its identity. As a result of this investigation, C. lacuskarlukiis transferred here to the genus Discostella. Discostella lacuskarlukiis 2.5–7 µm in diameter, has a shallow mantle, a stellar pattern of alveoli in the valve centre that may be reduced or completely absent, bifurcating costae and 4–5 marginal fultoportulae opening externally by short thickened tubes with round openings. We observed this species in numerous lakes across Alaska, in Lake Tahoe, Nevada, in several lakes across Canada, and in tundra and tree-line lakes of Northeastern Siberia. Although direct comparison with other Discostellaspecies is difficult because the small size of these diatoms requires high-resolution imagery to reveal ultrastructural details, it appears that D. lacuskarlukiis conspecific with D. tatricaProcházková et al. recently described from lakes of Central Europe and also reported from the European part of Russia. It is also conspecific with D. nipponica(Skvotzov) Tuji & D. M. Williams known from Japan. The apparent wide distributional range of this species indicates that it has been overlooked and commonly reported as D. pseudostelligeraor D. stelligerain arctic, boreal and alpine lakes.