From editor or global species database
Type species Sars included two new species, S. cochleatum and S. intestinale, one of which must be the type, whereas the type species is given as Streblosoma bairdi (was Grymaea bairdi Malmgren 1866) in the Hartman catalogue (p.522), and this is followed by Fauchald, 1977 and modern authors, presumably because it is regarded as the senior synonym to Streblosoma cochleatum. However, only taxa as originally included in the genus can be type and the type species name is set by nomenclatural rules and is immutable, although the name (as later assigned by taxonomic opinion) may now be an unused synonym.
[further note added March 2017 - Hartman clearly was indirectly indicating Streblosoma cochleatum as the type of the genus as she records the type as S. bairdi and has S. cochleatum as its synonym. Verrill (1900: 661) had earlier pointed out that Grymaea was a junior homonym to a protozoan (Grymaea Fresenius, 1858) and replaced it with Streblosoma, and designated (thus by subsequent designation) the type of Streblosoma to be S. cochleatum. [details]