Hello…my first post, but i’ll keep the questions coming as they come to me. I am wondering why some nouns (dux, ducis; lux, lucis) are not i-stems while others with the same form are (arx, arcis). I had thought that those three words were not i-stems because they didn’t take -tis in the genitive (nox, noctis; ars, artis), but arx, arcis defeated that rule. Can anyone clear this up? Also, if anyone has a list of the 3rd declension nouns like dux, ducis that aren’t i-stems (assuming there aren’t too many exceptions), i would appreciate it.
Hi whalenburns and welcome.
Salve whalenburns et gratus nobis tuus adventus.
Compiled using my Verba software (based originally on Whitaker’s Words data). Please advise, others, if there are mistakes.
Data per programma meum Verba nomine composita (quod programma in data ex Words apud Whitaker conditum est). Si errata animadvertistis, sodales, me moneatis, quaeso.
3rd declension nouns ending in x // Nomina tertiae declinationis per x litteram terminantia
Consonant stems // radices per consonantem
Masculine gender // masculini generis:
addax, adflictrix, afflictrix, Ambiorix, anthrax, apex, aquilex, aruspex, aspalax, aureax, aurifex, aurufex, berbex, bervex, box, calix, calyx, carnifex, carnufex, caudex, ceryx, ceyx, chalazophylax, chartophylax, chenalopex, Cingetorix, cnodax, coccyx, codex, corax, cordax, culex, culix, cyix, dapifex, decunx, dentex, dentix, dentrix, deunx, Dumnorix, dux, elix, Eporedorix, executrix, exsecutrix, extispex, fornix, frutex, haruspex, ibix, interrex, judex, lanx, latex, meddix, murex, onyx, opifex, opopanax, Orgetorix, oryx, panax, pantex, phoenix, podex, pollex, Pollux, pontifex, pontufex, pulex, pumex, quincunx, ramex, remex, rex, senex, septunx, sorex, storax, sturax, styrax, supplex, testatrix, thorax, Thraex, Thrax, Threx, tradux, Traex, Trax, Trex, Vercingetorix, vertex, vervex, vervex, vindex, vorax, vortex, irpex, hirpex, Felix, Beatrix, Ajax
Feminine gender // Feminini generis:
acceptrix, accusatrix, acquisitrix, actrix, adjutrix, admonitrix, adpendix, adquisitrix, adsentatrix, adsestrix, adsistrix, adultrix, adversatrix, aex, agitatrix, alopex, altrix, ambestrix, ammonitrix, anacampserox, appendix, arbitrix, argutatrix, arvix, assentatrix, assistrix, atriplex, balineatrix, balneatrix, balux, bellatrix, Bibrax, calabrix, calcendix, calumniatrix, cantrix, carex, cavillatrix, celox, cervix, choenix, cicatrix, circulatrix, circumspectatrix, cistellatrix, clacendix, coix, commendatrix, competitrix, compotrix, concitatrix, conditrix, conjectrix, conpetitrix, consectatrix, conservatrix, consiliatrix, consultrix, consupplicatrix, contemptrix, contemtrix, cornix, coturnix, coxendix, creatrix, creditrix, crux, cultrix, cylix, debitrix, defenstrix, deletrix, dominatrix, donatrix, educatrix, effectrix, expultrix, faex, fax, fellatrix, filix, forfex, fornax, forpex, frux, furnax, genetrix, gubernatrix, habitatrix, harvix, ilex, imitatrix, indagatrix, interfectrix, inventrix, janitrix, kalumniatrix, lectrix, lex, lodix, lux, matrix, mediatrix, meretrix, moderatrix, natrix, nix, nutrix, nux, opstetrix, oratrix, ornatrix, paelex, palux, pax, pelex, pellex, phalanx, pistrix, pix, potrix, praecantrix, praevaricatrix, prex, radix, rebellatrix, receptix, rumex, salix, saltatrix, sandyx, servatrix, solox, spectatrix, Sphinx, struix, supellex, tamarix, textrix, tomix, tonstrix, venatrix, venditrix, victrix, viviradix
Common gender // generis communis:
artifex, artufex, auspex, bombyx, calx, cojux, conjunx, conjux, cortex, grex, illex, index, inlex, limax, lynx, obex, perdix, silex, varix
Neuter gender // generis neutrius:
alex, allex, atriplex
I-stems // radices per i
Masculine gender // masculini generis: calx
Feminine gender // Feminini generis: arx, falx, faux, merx, nox
Common gender // communis generis: none // nullum
Neuter gender // Neutrius generis: none // nullum
Hello!
I’ll try to answer this one 
. I think it depends on where the noun comes from. For a noun like “dux, ducis” or “rex, regis” it’s clear that it’s a consonant stem. But I’ve also learnt that there are some “mixed” nouns that work a bit like consonant stems and a bit like i-stems. We call them “mixtos” in Spanish (which means “mixed”).
The rule they gave us is: “A noun is “mixed” when it has two consonants before the genitive in -is, and when it “seems” (seems but is not!) like a consonant stem.”)
Actually look at the i-stems ending in -x adrianus gave you:
calx, arx, falx, faux, merx, nox
These presumably have genitives like these: calcis, arcis, falcis, (faucis?), mercis, noctis
The only one breaking my rule is faucis, a word I’ve never seen before… good enough 
! The remaining ones have all of them two consonants before their genitive -is.
“Mixed” nouns like these happen to make you remember words like “dux” because they end in -x as well (contracted from -cs or -cs < -cts).
Other “mixed” nouns will end in -s in the nominative instead of -x because there’s no graphical contraction, like “mons, montis” or “dens, dentis” Haven’t you wondered why these are i-stems and “virtus, virtutis” isn’t? My rule here applies too.
So until you get a taste of which nouns are of what kind, I think if you follow that rule you won’t have many problems. (Though probably someone can find an exception.)
Hope I didn’t get it wrong and it helped
Though the rule isn’t exactly hard and fast, the word arx doesn’t defeat it. It’s not the t in the stem that’s important, but the number of consonants at the end of the stem, viz. two consonants in arx, arcis; nox, noctis; merx, mercis, etc., but only one in dux, ducis; lux, lucis; rex, regis; etc.
As per Adrian’s list, faux is an exception.
“Faux” is the only one in the list (that I can see; I didn’t examine the list extremely closely) that has a diphthong before the “x”, so that might be the reason.
It would seem another exception is “-nx” words, which become “-nc” or “-ng” in the genitive (maybe other things, too; I haven’t checked). Phalanx, phalangis – yet not i-stem. It might just be that they’e not i-stem because they’re more than one syllable, whereas all the i-stem words are monosyllabic.
Words that end with -s will be i-stem, too, when the stem ends with two consonants. For example, dēns, dentis.
Note also nix, nox, strix (and possibly more words ending in x) are mixed. My routines don’t handle them as such (I forgot about that category).
Nota quoquè, nix, nox, strix nomina admixta sunt, et praetereà fortassìs alia verborum in x terminantium. Ea programmate meo non sic tractantur (illius generis oblitus sum).
I don’t know why I didn’t pay attention to the two consonants - I had just read over that paragraph in wheelock’s while trying to figure it out. It didn’t get absorbed.
Allen and Greenough treats i-stems pretty exhaustively. See here.