[tied] Re: substratums

I thought I had the answer to the question as to why we don't
see the Latin plural of "virus" used -- that it is fourth declension
so the nom. pl. is spelled like the nom. sing. But wrong again!
Checking my Allen & Greenough Latin grammar, I find it's
second declension. But my vague memory of something odd
was right -- "virus", with "vulgus" (usually) and "pelagus" are
neuter rather than masculine.

"Pelagus" is Greek, but what's the explanation for the anomaly
with "virus" and "vulgus"?

Dan Milton