What is the status of words of Latin extraction which end in -um in the singular and -a in the plural? This seems to be an eroding contruction. The word "Conundrums" has recently been brought to my attention. My sincere thanks to all who could weigh in on this subject.

Uh, not sure what you mean by status. They're nouns that usually take either an -a or -ums in the plural. Borrowing a word is one thing, but borrowing a language's morphology is another. There are also lots of Greco-Latin loanwords that have plurals which are confusing or non-obvious to people who haven't studied classical languages: e.g., virus, apparatus, octopus, opus, etc.

Words that have an active life in the singular, e.g., conundrum, will be treated as English words and will eventually be given English plural forms. Others, such as data, that are rarely, if ever, heard in the singular, or which have a significantly different meaning in the singular, such as medium/media will be treated as a group singular in the plural form and have an English plural for the singular form's meaning. One, opera, which has taken on a specialized meaning, has become so divorced from the singular (opus) that it has its own plural, operas.

Thank you, zmjezhd. By "status" I suppose I meant accepted usage. I was taught that "-a" was the proper way to make a "-um" noun plural. It seems everywhere I look, however, an "s" is being tacked on to words such as "memorandum", "addendum", "conundrum", etc. Any thoughts?

I think it's as Faldage said, that words which are well-used in English acquire English plurals. Two that come daily into my sphere are "aquarium" and "terrarium". I am still divided on whether "aquaria" or "aquariums" is correct, but I gotta say most of my customers use the latter. I suspect that the "ums" ending will eventually dominate.

Though I tend to use the Latin plurals, I wouldn't go so far as to call the English ones wrong. People usually give me a funny look when I refer to a single graffito. I'm not sure what folks would say if I suggested: Let us repair (to the) solario. Rather than to the solarium. Not even sure that the English to should take the Latin dative, or whether the to the should be dropped. Hmmm.

It is not incorrect to use "stadiums" nor "referendums" but you will be more highly esteemed and probably get a couple of weeks off from Purgatory if you use "stadia" and "referenda" instead.

and let's not forget octopi and hippopatamuseseses.

Purgatory

Null where void by law and/or common sense.

Purgatory
Null where void by law and/or common sense.

XXII. Of Purgatory.
The Romish Doctrine concerning Purgatory, Pardons, Worshipping and Adoration, as well of Images as of Relics, and also Invocation of Saints, is a fond thing, vainly invented, and grounded upon no warranty of Scripture, but rather repugnant to the Word of God.

~The Articles of Religion

and let's not forget octopi and hippopatamuseseses.

According to a Greek-speaking friend of mine, "octopus" takes a Greek plural - "octopodes" - and so she recommends "octopuses".
Eta, do you ever have trouble figuring out how to stop spellin chrysanthemumumumumum?