Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/849

 bone is unusually thick, reaching 2| inches at the angle ; and the articular surface is correspondingly broad from the dorsal to the ventral margin. The proximal end bears a subhemispherical articular head (figs. 1 & 2, a), the axis of which makes an angle with that of the cylindrical shaft. A groove passing between the pre- and postaxial borders separates this articular caput from a prominent, non-articular, narrow, trochanteric upward production of the dorsal surface of the bone (fig. 2, b). The trochanter (mutilated in our specimen) descends a short distance along the postaxial border. The groove which separates this from the articular caput descends along both borders, on the postaxial one inclining to the ventral surface along the prolongation of the trochanter. A constriction below the articular head marks off on the ventral surface a smooth neck, beyond which the whole breadth of this surface and also the postaxial border of the cylindrical part of the humerus are occupied by a very rough and swollen muscular impression. The expanded distal part of this surface is smoother. From the preaxial border transversely, this surface is gently convex to the wing, where there is a shallow longitudinal hollow. The dorsal surface is smoother than the ventral. A low eminence appears at about one-third of the length of the bone, measured from the upper edge of the trochanter. I am unable to select the other bones of this fore paddle.

The femur (fig. 3) is rather longer than the humerus, and it is also less massive. This is particularly noticeable in the shaft. It has a subglobular articular head (figs. 3 & 4, a) placed obliquely on the shaft, separated, as in the humerus, by a groove from a very largely developed trochanter (fig. 3, e, & fig. 4, b), which projects strongly postaxially, and descends on this border of the shaft. The groove also descends on both borders of the bone. It is deeper than the corresponding groove in the humerus. The articular caput is followed by a smooth constricted neck, to which succeeds, on the ventral surface and postaxial border a large rough warty swelling. The preaxial border (fig. 3, c) is concave, not straight as is that of the humerus ; and the postaxial border (d) is still more concave, its distal half curving strongly backwards, and making an extremely large wing with the distal border. This last border is very remarkable : its outline consists of three distinct segments, corresponding to as many divisions of the distal articular surface, which is one in excess of those present in typical Plesiosauri. The preaxial and the next postaxial division of this surface meet in a large angle lying just behind the axis, as in the humerus. Behind this is a second postaxial division of the articular surface, answering to the wing. The greater part of this, the third division of the distal articular end, is wanting ; but its form is perfectly given by that of the corresponding bone in the second segment of the paddle.

This segment of the hind limb, the equivalent of the leg, contains in a transverse row three flattened angular bones of nearly equal size (f, g, h), each of which articulates proximally with one of the divisions of the distal articular surface of the femur, and distally with the members of the succeeding segment of the paddle. Laterally the

2z2