Are you looking for "Jaguar XK120C based prototype nicknamed the Brontosaurus".
I'm not sure whether it had an XK120 chassis or a C-Type chassis, I've seen both suggested. One fairly authoritive website lists it as XKC301 suggesting a C-type (or possibly early D-Type) chassis.
It's difficult to say what this was a prototype for. It comes between XKC201 and XKC 401 in the chassis numbering sequence. XKC201 was the "C-Type Mk 22 or "C/D-Type prototype" produced in early 1953 by the Competitions Department and XKC401 was the first D-Type produced in 1954 so it wasn't for that. It came later than the unsuccessful 1952 Le Mans cars and has a very different body from them. The "Brontosaurus" was apparently produced in late 1953 directly for [Sir] William Lyons and not by the Competitions Dept. It had spats on the front wheels as well as the rear ones so its turning circle was too large for track use. When asked by Norman Dewis, apparently Lyons suggested that it could be used for record breaking. But it never was.