I think it's probably a surname; I had a look at the post below and, as it's a middle, I'm guessing it's a family surname.
I like
Perdita "Perdie".
I don't think
Arabella,
Allegra, or
Myrtle are becoming popular, they're only being used in narrow circles, I believe.
According to the 2013 data for the
UK, the latest available:
Arabella is at #157
Allegra is at #772, sharing the spot with 11 other names: Aroush,
Eira,
Elina, Kaci, Khaleesi,
Peggy, Rahma, Ruby-May,
Selina,
Tori, and
Viktoria.
Myrtle is at #3536, which doesn't mean much as it was only used six times.
I can easily see usage of
Arabella having increased in the past year, but less so the other two.
Ottilie

http://www.behindthename.com/pnl/116467