Håvard said:
>cannot one
>assume passive ending -()ris in third conjugation as analogical to
>the same found in 1st and 2nd conjugations?
Yes - but in all four conjugations, it is only coincidence that makes them
look like the passive infinitives. There is no reason to think they derive
from anything except the older inherited forms in -so, with an -s often
added to clarify the person ending.
The forms in -so are secondary , but are nonetheless derived from PIE
(shared by Latin, Greek and Skt)
The Latin forms in -ris appear around about 200 BC, and become more and more
frequent. By 1AD they are much more common than the forms in -re.
Peter