This name is a synonym of Crataegus crus-galli var. salicifolia (Medik.) Sol..
The record derives from RJP
(data supplied on 2011-10-18)
which reports it as a synonym (record 14698) with original publication details: Synopsis plantarum Synopsis plantarum; 1805 37 1806.
This name record (from RJP) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Crataegus linearis in version 1 of The Plant List.