[BioC] locate a target species in Refseq ftp directory

[heyi xiao]

Hi all,
I am trying to extract the RNA sequences for sheep (or Ovis aries) in Refseq ftp site. The right directory should be vertebrate_mammalian: ftp://ftp.ncbi.nlm.nih.gov/refseq/release/vertebrate_mammalian/
But there so many *rna* files there, all named with some numbers, like vertebrate_mammalian.154.rna.fna.gz, not sure which one is for my target species. Readme files don’t really help on this. does anyone knows how to locate the right file for a target species there?
Heyi