Just found this - http://www.edinphoto.org.uk/pp_n/pp_pettigrew.htm about Pettigrew and Amos

I think the photos may be cartes de visite - they look particularly like the one of the man near the bottom of the page

Edit -looking at this http://en.wikipedia.org/wiki/Carte_de_visite , they are too small to be cartes de visite, as they are about 4.3" x 6.3" - I think they are cabinet cards
http://en.wikipedia.org/wiki/Cabinet_card

That would make them albumen prints http://en.wikipedia.org/wiki/Albumen_print , I think