Interesting and intriguing slide this. It’s an Aulacodiscus sp. from Oamaru, New Zealand. Single example on the slide. Prepared by Ernest Wainwright in what I think reads 1942. The following is written on it ‘altho[ugh] family common in Oamaru this Aulac[odiscus] has never had a sp[ecies] name’. So, this is an Aulacodiscus of some type, but he thought it didn’t have a species name. I’ve looked through images and descriptions of Aulacodiscus from Oamaru, and the closest match I can find is Aulacodiscus margaritaceus var. undosa (I’ve also seen it written as undosus). Rattray, J. (1888). A Revision of the genus Aulacodiscus Ehrb. Journal of the Royal Microscopical Society, 1888: 337-382, 3 pls. described it as;
Var. undosa, Grove & Sturt MS. Diam. 0.2 to 0.445 mm. Surface, central portion flat, stellate with angles at processes, and sides deeply concave between these. Markings 2 in 0.01 mm. Primary rays distinct. Processes 9 to 14, insertion 1/9 to 1/11 of radius from circumference, obconical, free ends but little rounded. The scar of the broken – off process is obovate. Habitat: Jackson’s Paddock, Oamaru (Grove !).
The overall description is a good match, however this one only has 8 processes. The image of the one on the Oamaru Diatoms website, only has 6 processes however, and I suspect this one probably is Aulacodiscus margaritaceus var. undosa with a fewer than usual number of processes. I will however leave a question mark after it for now.
Olympus BHB microscope using 450nm LED light. 20x Nikon Plan Apo NA 0.65 objective (big diatom, diameter as measure in ImageJ, 283.3µm). Olympus Aplanat Achromat condenser, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 46 images stacked in Zerene (Pmax).