dieses; ellipsis, ellipses; emphasis, emphases; fascis, fasces; hypothesis, hypotheses; metamorphosis, metamorphoses; oasis, oases; parenthesis, parentheses; phasis, phases; praxis, praxes; synopsis, synopses; synthesis, syntheses; syrtis, syrtes; thesis, theses.  In some, however, the original plural is not so formed; but is made by changing is to _~ides_; as, aphis, aphides; apsis, apsides; ascaris, ascarides; bolis, bolides; cantharis, cantharides; chrysalis, chrysalides; ephemeris, ephemerides; epidermis, epidermides.  So iris and proboscis, which we make regular; and perhaps some of the foregoing may be made so too.  Fisher writes Praxises for praxes, though not very properly.  See his Gram, p. v. Eques, a Roman knight, makes equites in the plural.

5.  Of nouns in x, there are few, if any, which ought not to form the plural regularly, when used as English words; though the Latins changed x to ces, and ex to ices, making the i sometimes long and sometimes short:  as, apex, apices, for apexes; appendix, appendices, for appendixes; calix, calices, for calixes; calx, calces, for calxes; calyx, calyces, for calyxes; caudex, caudices, for caudexes; cicatrix, cicatrices, for cicatrixes; helix, helices, for helixes; index, indices, for indexes; matrix, matrices, for matrixes; quincunx, quincunces, for quincunxes; radix, radices, for radixes; varix, varices, for varixes; vertex, vertices, for vertexes; vortex, vortices, for vortexes.  Some Greek words in x change that letter to ges; as, larynx, larynges, for larinxes; phalanx, phalanges, for phalanxes. Billet-doux, from the French, is billets-doux in the plural.

6.  Of nouns in on, derived from Greek, the greater part always form the plural regularly; as, etymons, gnomons, ichneumons, myrmidons, phlegmons, trigons, tetragons, pentagons, hexagons, heptagons, octagons, enneagons, decagons, hendecagons, dodecagons, polygons.  So trihedrons, tetrahedrons, pentahedrons, &c., though some say, these last may end in dra, which I think improper.  For a few words of this class, however, there are double plurals in use; as, automata or atomatons, criteria or criterions, parhelia or parhelions; and the plural of phenomenon appears to be always phenomena.