According to the seller on Ebay these drawings were executed at the Bruges Academy which was under the direction of Jozef van de Kerckhove, Matthias de Visch, and Jan Antoon Garemijn.
Skeleton from The Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers, par une Société de Gens de lettres by Denis Diderot and Jean le Rond d'Alembert. These plates are engraved by J.A.Defehrt and Bonaventure/Benoit-Louis Prevost after J.R Lucotte or Goussier. Another post about the Dictionary.And one more.
Skeleton drawing attributed to John Linnell, British portrait and landscape artist.
Here are some portraits by John Linnell at the Auckland Art Gallery.