A slide labelled as Melosira arenaria. It also says L.Mourne, which I am assuming is Lough Mourne (more on this is in a minute). No makers name but style and handwriting looks like some I have by JA Long. Low magnification image using a 20x objective showed the bottom right one to have quite a bit of surface texture in the middle. Theses ‘scars’ did not show up well at higher magnification though.
Olympus BHB microscope using 450nm LED light. Main image 63x Leitz Pl Apo NA 1.40 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, slightly oblique. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 14 images stacked with Zerene (Pmax).
There is an article about diatoms from Lough Mourne; XIV.—On deposits of diatomaceous earth, found on the shores of Lough Mourne, County Antrim, with a record of species living in the waters of the lake, by Smith, W., 1850, The Annals and magazine of natural history; zoology, botany, and geology, 5(2), 121-125. Online copy – https://www.biodiversitylibrary.org/page/18687143#page/135/mode/1up. Melosira arenaria does appear in this article but no images are given.
This is also known as Ellerbeckia arenaria (see here).