The word "parexus" is a bit of a mystery when it comes to spelling. However, the IPA phonetic transcription can shed some light on how it's spelled. Pronounced /pəˈrɛksəs/, the word begins with the "p" sound, followed by the short vowel sound "uh." The next syllable is pronounced with the long "eh" sound followed by the "ks" cluster. Finally, the word ends with the "uh" sound and the "s" sound. So, while the spelling may be a little unclear, the IPA transcription makes it easier to understand the correct pronunciation.
In geol., a genus of fossil fin-spined fishes of small size found in the lower Old Red Sandstone.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.