The illustration in your post looked familiar to me I knew I had seen it somewhere, and now I remember!
It was in the article:
The Earliest Discoveries of DinosaursJustin B. Delair and William A. S. Sarjeant
Source: Isis, Vol. 66, No. 1 (Mar., 1975), pp. 5-25
Published by: The University of Chicago Press on behalf of The History of Science Society
In this article, there is the same illustration that you posted and a little quote from Plot:
I have one dug out of a quarry in the Parish of Cornwell, and given me by the ingenious Sir Thomas Pennyston, that has exactly the Figure of the lowermost part of the Thigh-Bone of a Man or at least of some other Animal, with capita Femoris inferiora, between which are the anterior (hid behind the sculpture) and the large posterior Sinus (the seat of the strong Ligament that rises out of the Thigh, and that gives safe passage to the Vessels descending into the Leg): and a little above the Sinus, where it seems to have been broken off, shewing the marrow within of a shining Spar-like Substance of its true Colour and Figure, in the hollow of the Bone, as in Tab. 8 Fig. 4. In Compass near the capita Femoris, just two Foot, and at the top above the Sinus (where the Thigh-Bone is as small as any where) about 15 inches: in weight, though representing so short a part of the Thigh-Bone, almost 20 pounds.
I have the article by Delair and Sarjeant in PDF file is anyone wants a copy just email me bj25 at le.ac.uk and I'll relpy with attached file asap.