Journal instructions for citing websites vary but typical citations might be:
Young, J.R., Bown P.R., Lees J.A. (2017) Nannotax3 website. International Nannoplankton Association. Accessed 21 Apr. 2017. URL: http://www.mikrotax.org/Nannotax3
Young, J.R., Bown P.R., Lees J.A. (2017) 'Cylindralithus' Nannotax3 website. Accessed 21 Apr. 2017. http://www.mikrotax.org/Nannotax3/index.php?taxon=Cylindralithus&module=Mesozoic
Including the date on which a page, on the site, was viewed is strongly advised, since the site is liable to change. Our current software does not archive old versions of the website, so you should use an external archiving system to record the specific content of the site when you accessed it. You can do this by using a service such as archive.is to create a permanent archive copy of any page from mikrotax. This is probably the most robust way to cite individual pages and is ideal for use in peer-reviewed journals. It also will provide a short URL, e.g the WebCite URL for the Helicosphaeraceae page as cached on 29th June 2017 is https://archive.is/w5r3S. Using this the citation would be of this form:
Young, J.R., Bown P.R., Lees J.A. (2017) 'Helicosphaeraceae' Nannotax3 website. 29th June 2017. https://archive.is/w5r3S
N.B. We previously reccomended Webcite however this website does not archive images and does not seem to be being actively developed (although archived pages are still available from it), so archive.is seems a surperior alternative.