Annals of the South African Museum

and slightly convex facet; its anterior border is sharp and almost straight, while the post-axial border is concave in its proximal half but distally is occupied by a large facet for union with the fibula, with which there seems to have been no contact at the proximal end. Distally there is a long, flat facet for the tibiale; in front of this there is a small, oblique facet and behind it a longer, also oblique facet for the intermedium. The fibula (fib.) is a kidney-shaped bone; proximally it bears a large, nearly straight surface for union with the femur; anteriorly it has in its proximal portion a concave,

4. Portion of hind paddle of Plesiosaurus capensis. About nat. size.
fem., femur; fib., fibula; tib., tibia.

sharp-edged border and distally a facet for union with the tibia. The remainder of the border is occupied by four facets-one for the intermedium, behind this a surface for the fibulare, next a postaxial surface looking nearly directly backwards, and lastly a small surface looking upwards and backwards, which probably supported an accessory ossicle which also articulated with the femur. The chief peculiarity of this tibia and fibula, apart from their length, is that, although they unite distally, proximally there is a considerable interval between them which is only closed above by the humerus. I am not acquainted with any Plesiosaur in which exactly the same thing is seen; usually in the post-Liassic Plesiosaurs the tibia and

fibulæ are much shortened, and if there is any interval between them, it is a small foramen or slit which is enclosed above and below by the meeting of these bones.

For comparison of this species with previously described species from the same or approximately the same horizon the cervical vertebræ are of the greatest importance, since the skull is quite unknown in the other species and only a few odd limb bones have hitherto been found. The cervical vertebræ are very similar to those of some of the nearly contemporaneous forms, particularly in the presence of a sharply defined and deep central depression in the middle of the more gently concave articular surfaces of the centra, but in all cases differences are apparent which forbid the inclusion of this form in any of the species hitherto described.

The forms which seem to resemble the present species most nearly are Plesiosaurus degenhardti Koken, P. limnophilus Koken, and P. valdensis Lydekker, sp.; there is also a certain amount of similarity to P. bernardi Owen and some other species.

In Plesiosaurus degenhardti* of Koken the form of the centra of the cervicals differs considerably, the height of their centra being greater than either their width or length; in the present species. the width is the greater, and at the same time in P. degenhardti the edges of the articular surfaces are more rounded and thickened and the zygapophyses are smaller. The difference in the proportions of the centra is shown if we compare centra of approximately the same size of the two species: thus, in the present species, if the length of the centrum be taken as 100, the width will be about 156, the height 138; while in P. degenhardti, the length being 100, the width will be 127, the height 152.

From Plesiosaurus limnophilus + of Koken the difference is also well marked, the length of the centrum being about equal to the width, while in P. capensis it is much less. In a vertebra from the Wealden of Cuckfield, Sussex, ascribed by Lydekker to P. limnophilus, but regarded by Koken as probably distinct, the length and height of the centrum are about equal, but the width is greater. Comparison with Plesiosaurus valdensis § shows a much more marked similarity; in this case, the length of the centrum being taken as 100, the width is 151, the height 129-proportions which

Koken, Palæont. Abhandl., vol. iii. (1887), p. 414; see also vol. vii. (1896), p. 122, pl. iii.

Koken, op. cit., vol. iii. (1887), p. 417.

Lydekker, Catal. Foss. Rept. Brit. Mus., pt. ii. (1889), p. 224.

approach very nearly to those of some of the vertebræ here described. In both cases also there is a sharp hæmapophysial ridge with a concavity on each side, and the articular surface is strongly concave with a sudden depression in the middle; this last character is more strongly marked in the present species. The neural spines in Plesiosaurus valdensis also resemble those of the present species, being rather low and curved backwards (see Lydekker, Catal. Foss. Rept. Brit. Mus., pt. ii., p. 188, fig. 61). There are, however, a number of differences in detail which seem to render it impossible that our specimen is specifically identical with this species.

Comparison of the proportions of the cervical vertebræ of different species of Plesiosaurs is only of value in a very general way since the vertebræ have been, for the most part, found isolated or in small groups, so that their position in the neck is usually uncertain, and they may have belonged to animals of different sizes. Nevertheless, on the whole it may be taken as fairly certain that Plesiosaurus capensis is a member of the group of small Plesiosaurs which in Europe is represented by P. degenhardti and P. valdensis from the Wealden, and perhaps by P. bernhardi from the Cambridge Greensand.

The occurrence of a Plesiosaur in South Africa is of peculiar interest since it is not improbable that the group may have been derived from some form related to the Therocephalia remains of which are found on the Permian of that region.

The approximate dimensions (in centimetres) of the type specimen of Plesiosaurus capensis are:

SKULL.

Length from tip of snout to occipital condyle

Length from tip of snout to hinder point of quadrate

Length from tip of snout to vertex

Length from tip of snout to external narial opening..

Width between outer angles of quadrates

Width of snout where the maxillo-premaxillary suture crosses the alveolar border ..

27.7

32.6

29.3

10.4