A stria in the shape of an elongated chamber. The external wall of the alveolus is perforated by many areolae and the internal wall is perforated by one long opening. Plural form is alveoli. An example genus is Pinnularia.
Cox, E.J. (2011). Morphology, cell wall, cytology, ultrastructure, and morphogenetic studies. Overview and specific observations. In: The Diatom World (J. Seckbach and J.P. Kociolek, Eds,), Cellular origin, life in extreme habitats and astrobiology, Vol. 19, pp. 21–45, Springer, Dordrecht.
Cox, E.J. (2012). Ontogeny, homology, and terminology - wall morphogenesis as an aid to character recognition and character state definition for pennate diatom systematics . J. Phycol. 48: 1–31. 10.1111/j.1529-8817.2011.01081.x
Round, F.E., Crawford, R.M. and Mann, D.G. (1990). The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge, 747 pp.