Terminology for members in series

[GaryUK]

I require some guidance on the correct terminology for the following:

 

In a series of numbers...

first / last members - ordinal / terminal (or other)?

next / previous members - ordinal / adjacent (or other)?

 

So far I have referred to them all as ordinal (as in 1st, 2nd, 3rd, ..., last), but I want to distinguish between first/last/next/previous.

I only ask because I wish to properly name methods in some program code I'm developing.

 

I am not a mathematician or statistician, etc, just a novice.

Thanks for any help.