"Josh" writes:

songs, that I wrote, from my last band.

my ex-bass player sent me this email:

"hey, [...] those are my bass lines and i have already
spoken to my attorney

no matter if you own the copyrights to the songs, those
are my bass lines.i don't have to have them protected by copyrights,
just simply have a recording with me as the bass player on the cd.

I would ask him to more clearly state his claim.
From his email, I can't tell if he thinks:

1. You alone did not write the song: it is a joint
work of which he is also the author.

2. He is the sole author of a song, comprised entirely
of those bass lines, to which you have no right.
He objects to your creating and performing a new
derivative work by playing them along with your song,

3. He owns the bass lines merely because he was the first
person to perform them, even though you composed them
and told him to play them for you.

He doesn't have any right to the music merely from having played it.
So he must be trying to say that you are fraudulently claiming
the copyright, or are infringing on his copyrighted song.
Simply put: he is claiming that he is the author, not you.

You may wind up in court, having to prove that you are the sole author
of the bass lines. The question will be who first fixed the music
into tangible form. If you wrote them down first (on a scrap of
paper or anything), and then showed them to him, you are almost
certainly the author. On the other hand, if you told him verbally
what to play, and then they were played by him into a tape recorder
before being written down, it could be a little more complicated.
One question will be who was agreed to be the owner of the recording.
One question would be about your exact relationship to him at the time.
Another question will be about the circumstances of when the song was
first set onto paper (such as who was there, how did your name get on it,
when did he become aware of this, etc.)

If he has already consulted with a lawyer who has advised him on
this matter, it seems very strange that he didn't get the lawyer
to write this first complaint letter for him. It is also incredible
that a lawyer would tell him that your copyright on the music in
question is not relevent. It is the only issue.

My guess is that he has not really presented the facts to any lawyer
who has actually given him advice. Maybe he ran some version of his
story by some lawyer (who may or may not actually know anything about
the entertainment industry or copyright law) on an informal basis,
and who gave him no advice to rely upon. By suggesting that this is
not about copyright, he is certainly mischaracterizing what any lawyer
would say, to such a degree that it's hard to believe him at all.
I'm no lawyer, but I can tell you that he has no right to the song
merely because he performed it on a CD. Performance rights (for you
or him or anyone to play the wong) belong entirely to the (copyright)
owner of the song.