"Certainly not neuter!"
While "|n" was copypasted and while I didn't look for the Latin gender before, "certainly" does not fit here:
- Different (New) Latin authors could have used different genders. Thus a masculine or feminine example, does not proof that trapezoides never was neuter.
- I've onced read that words in -oides have the gender of the original word, e.g. *asteroides would be masculine as the Greek word ἀστήρ astḗr is masculine. It wasn't a reliable source and I haven't checked this statement, but it could be correct. If it is, trapezoides would be neuter as trapezium and the original Greek word are neuter.
- Latin trapezium and German Trapez, Trapezium, Trapezion, Trapezoid and Trapezoides are neuter, and Trapezoides was declined like a Latin word (genitive Trapezoidis, dative Trapezoidi, ablative Trapezoide etc.). For Trapezoides one can also find the neuter plural "Trapezoida" (1786: "die Trapezoida", "gleich große Trapezoida", "drey Trapezoida"; 1733: "in Trapezia und Trapezoida").
Well, there could be gender-shifts from Latin into German, but they should largely be restricted to "irregular" or "unusual" words like changing feminine paragraphus into a masculine Paragraphus. But paragraphus was also masculine in New Latin, thus it should be doubtful whether or not there was a gender-shift when the word was adopted into German. In case of trapezoides/Trapezoides there should be no reason to assume that there was a gender-shift. - books.google.com/books?id=WZE3ms9681cC&pg=PT23 (1560) has "Trapezia, & Trapezoidea Euclides communi nomine Trapezia vocauit". That should say that Euclid used the single term trapezium for both trapezium and trapezoid, and trapezoidea should be a neuter plural, see -ειδής. books.google.com/books?id=3Tp96ebQX-sC&pg=PR33 (1619) also has the plural trapezoidea and writes "..., & trapezoidea omnia communi nomine appellauit trapezia ..." which could proof neuter gender (omnia, inflected form of omnis). books.google.com/books?id=NLhRAAAAcAAJ&pg=PA13 (1871) has a Greek and a Latin text and uses trapezoidea in Latin too.
So it's very likely that trapezoides is indeed neuter.
The plural however might be somewhat doubtful: "trapezoidea" can be found in earlier New Latin (1500-1650), while "trapezoida" can be found in German (18th century). As educated Germans back then learned Latin and wrote and read in Latin, it's likely that the authors would have also used trapezoida and not trapezoidea in a Latin text. But well, it does not attest the Latin plural trapezoida.
PS: One can also find trapezoidem and Trapezoidem (acc. sg.), e.g. "Trapezoidem in [picture of a triangle or a big delta] convertere." and "des Trapezoidis [...] den Trapezoidem". In German it's masculine, and I'd guess it's masculine in Latin too. But as stated before, this does not proof that the word was never neuter. -80.133.101.204 04:30, 24 August 2016 (UTC)
The difficulty is that nouns ending in -ēs were never neuter in Latin. In fact I don't even know if there is a neuter declension they'd fit into. So if this is neuter, it'd certainly be a one-of-a-kind exception.
Well, I just added "trapezoides" without any length. Someone else changed it into "trapezoidēs". Compared with other words ending in -oides the i should be long, that is "trapezoidēs" should be incorrect anyway.
Dictionaries mention several neuter nouns in -oides, like ascyroīdes, cynoīdes (or cynoīdēs), neuroīdes, sēsamoīdes - though some of them might be hapax legomena or words just used in science and not in poetry. In case of cynoīdes/cynoīdēs different dictionaries give different lengths for the e. But maybe there's no proof for any length. Also dictionaries mention adjectives in -oides, which are -oides for all genders in the nominative singular like monoides. But it could be monoīdēs, ēs, ēs (neuter with long e), or monoīdēs, ēs, es (neuter with short e like in Greek). There are also other adjectives in -es like diopetēs and īsoscelēs. But again the length of the neuter nominative singular might be doubtful, and maybe there's no proof for any length too and maybe it's just a guessing.
Based on dictionary entries it should be more likely that the neuter ends with a short e, e.g. for neuter nouns in -oides it's more often -oīdes in dictionaries than -oīdēs. That is, it should be trapezoīdes n (a and first e should be short like in τραπέζιον (trapézion) and τράπεζα (trápeza), plural should be trapezoīdea, maybe also trapezoīda). With accusative trapezoidem and plural trapezoides it should rather be trapezoīdēs m, but I have no Latin proof for the gender. Reasons to assume masculine instead of feminine gender: German Trapezoides with accusative Trapezoidem was masculine, and trapezoide and trapézoïde in Romance languages are masculine too.
As far as I know, trapezoīdes n is indeed a one-of-a-kind exception, but not because of its neuter gender, but because of the plural trapezoīdea which can be found in New Latin. For other words in -oides the neuter plural might be unknown, and in case of the adjective isosceles one can find the plural isoscelia. Side note: -oidea is also an inflected form of New Latin adjectives in -oideus like rhomboideus. That is, just finding a word form ending in -oidea regardless of context does not attest a neuter plural for a word in -oides. -80.133.101.204 13:53, 24 August 2016 (UTC)