Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/853

 digits. As often as I looked at the cast of this gigantic paddle (6-1/2 feet long), I was struck with the insufficient basal support of the hindmost digit, and the suspicion forced itself upon me that something was wanting in the postaxial side of the foot. The Portland paddle proved this to be the case. Its second segment contains two coequal bones, and part of the distinct impression of a third, which has fallen out of the matrix ; and corresponding to this impression in the stone there is, as may be plainly seen in the excellent figure illustrating Prof. Owen's memoir, the beginning of a second postaxial facet on the distal end of the femur. The rest of this facet, with the adjacent postaxial border, has been worn away. Prof. Owen regards the third ossicle in the enemial row as " the homologue of the fabella which is present in some Plesiosauri (P. rugosus, for example), where its homotype in the fore limb is represented by a detached olecranal process of the ulna," adding " that the bone in Pliosaurus portlandicus is relatively larger and less triangular in shape than in Plesiosaurus rugosus." Afterwards, describing the hindmost of the three bones present in the first row of the tarsus, which he regards as the calcaneum, Prof. Owen writes of its posterior border, " It appears to have been closely connected with the fabella (67'), which fits into the interspace between the fibula and the calcaneum ; and whether to regard the ossicle marked 67' as the apophysial lever of the fibula or as the calcaneum may be a question"*.

The identity of the third enemial bone of our Kimmeridge Plesiosaur with that which has left its impression in the matrix in which lies the Portland Pliosaurus-paddle cannot, I think, be questioned ; and the correspondence of this to the ossicle attached to the posterior border of the fibula of Plesiosaurus rugosus, pointed out by Prof. Owen, appears highly probable. If this be a true correspondence, and not simply an apparent one, the determination of the serial homology of the bone is still open; for no confirmation of either of the two hypotheses is to be drawn from the Kimmeridge paddle. If the ossicle attached to the fibula in Plesiosaurus rugosus is the homologue of the " apophysial lever of the fibula " (the styloid process of human anatomists), we have the anomaly of its lateral, not terminal, articulation with the shaft of the fibula, and also of its articulation with both the femur and the tarsus. If, however, it is the calcaneum, we have another anomaly — the articulation of the heel-bone and femur — besides the difficulty of reconciling this determination with that of the hindmost of the three bones of the first tarsal row, which the author of the memoir quoted regards also as the calcaneum.

We have not yet, I think, sufficiently complete material for the solution of this problem, and we must be content to wait for fresh material before the homology of the bone can be settled. At present, I will merely say that its existence as a coequal bone with the fibula and tibia in the Kimmeridge Plesiosaurus is in harmony with the ingenious theory of the type of limb-construction recently advanced


 * Paleont. Soc. vol. xxii. ' Fossil Reptilia of the Portland Stone,' p. 11.