There are valid reasons for doubting the reappearance of the intermedium after its loss in the terrestrial ancestors of the plesiosaurs. It may be the enlarged centrale. The bones in the distal row may be the first, fused second and third, and the fourth tarsalia. The homologies of the mesopodial bones of the Ichthyosauria (Fig. 158 c, d), where a like similarity between the front and hind limbs exists, are even more doubtful. There is the same objection to the recognition of an intermedium tarsi in this order as in the plesiosaurs, whatever may be the corresponding bone in the carpus.
Fig. 159. Paddles of Plesiosaurs: A, right hind paddle of Thaumatosaurus, after Fraas. B, right hind paddle of Trinacromerum. C, right front paddle of same individual. f, femur; fb, fibula; t, tibia; h, humerus; r, radius; u, ulna.
