Recent molecular studies have shown that Boletus in its current circumscription is likely an artificial grouping and it is possible that it will be split at some point into smaller genera. Note that Boletus impolitus and Boletus depilatus for practical reasons are retained here, although there is strong evidence that they are closely related to Xerocomus subtomentosus and its allies.
Fruitbody large to medium sized, boletoid, without veil and ring. Stipe solid, with surface usually covered with granules or network. Flesh variously coloured, changing or not when exposed to air. Tubes easily separable from each other, not tearing apart. Pores usually small and rounded.
Boletus caucasicus Singer ex Alessio [nom. inval.]
An enigmatic bolete, very similar to Boletus luridus and distinguished by the stipe, which is covered with network in the upper part and granules in the lower, the yellow (and not reddish) flesh under the tubes and the inamyloid hyphae in the base of the stipe. The name B. caucasicus has never been validly published and for the time being it is better considered dubious. For extensive discussion see Vizzini & al. (2014).
There are no identifiable photographs available to me or featured online. Colour paintings are featured in Engel & al. (1983) and Alessio (1985).
Alessio, C.L. 1985. Boletus Dill. ex L. (sensu lato). – In: Fungi Europaei. Vol. 2. Pp. 1–705. Libreria editrice Biella Giovanna, Saronno.
Engel, H., Krieglsteiner, G., Dermek, A. & Watling, R. 1983. Dickröhrlinge. Die Gattung Boletus in Europa. Verlag Heinz Engel, Weidhausen b. Coburg.
Singer, R. 1967. Die Röhrlinge. II. Die Boletoideae und Strobilomycetaceae. – In: Die Pilze Mitteleuropas. Vol. 6. Pp. 1–151. Julius Klinkhardt Verlag, Bad Heilbrunn.
Vizzini, A., Simonini, G., Ercole, E. & Voyron, S. 2014. Boletus mendax, a new species of Boletus sect. Luridi from Italy and insights on the B. luridus complex. Mycological Progress 13: 95–109. (available online)