Pleurothallis, abbreviated Pths in horticultural trade, is a genus of orchids commonly named Bonnet Orchids. The genus name is derived from the Greek word ‘pleurothallos’, meaning ‘riblike branches’. This refers to the rib-like stems of many species.
This was a huge genus, which used to contain more than 1,200 species – the second largest in the Orchidaceae after Bulbophyllum. In 2004, it decreased by more than half when many species were moved into new genera (Monogr. Syst. Bot. Missouri Bot. Gard. 95: 256. Feb 2004).
Reference : http://en.wikipedia.org/wiki/Pleurothallis