In 1842, both G. B. Sowerby I and G. B. Sowerby II were actively
involved in malacology.  Sowerby I would have been in his mid-50's.
Sowerby II would have been 30 years old (born in 1812).  G. B. Sowerby
III would only have been a twinkle in the eye of Sowerby II.  Don't
assume it is Sowerby I when it just says "Sowerby".  All three of them
are often listed without the Roman numeral.

  Paul M.