Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/851

 sired. From the figure of the conjoined axis and atlas of the Haddenham specimen, given by the late Mr. L. Barrett in ' Annals & Mag. of Nat. Hist.' 1858, and from some sketches of the coracoids, and of a restored paddle of the Ely one, there are evidently strong resemblances ; but Mr. Davies, sen., of the Palaeontological Department of the British Museum, whose opinion in a matter of this kind is justly entitled to great weight, tells me that he has seen the Haddenham spinal column, and believes it to represent a distinct species from Mr. Mansel's.

A comparison of the limbs of Mr. Mansel's, and also of P. megadeirus, with those of the typical Liassic Plesiosauri, brings out so many and such important differences that these two Kimmeridge Enaliosaurians may well rank as the representatives of a very distinct subgenus.

Their coracoids are not produced forward mesially far in advance of the level of the glenoid cavity as they are in the Liassic Plesiosauri, but their anterior borders make a nearly straight line. They are also much extended transversely to the trunk's axis, while their greatest extension in the Liassic Plesiosauri is in the direction of the axis. In this the coracoids of the Kimmeridge Plesiosauri show a resemblance to Ichthyosaurus ; but their front border wants the characteristic Ichthyosaurian notch.

The humerus and femur of Mr. Mansel's Plesiosaurus are absolutely, as they are also relatively to the length of the spinal column, longer than in any known Liassic Plesiosaurus. They are also more massive, and their muscular impressions are much stronger. Their subglobular articular caput appears to me to make a larger angle with the axis of the bone ; and there is also the well-developed trochanter. The postaxial border has a larger backward curve ; and the wing is much larger. The distal end of the femur (that of the humerus is incomplete ; but by analogy it should resemble that of the femur) has a third articular facet, while that of the humerus and femur of Liassic Plesiosauri has two articular facets only. The second segment of the limb, the enemion, contains three coequal bones, articulating with the three femoral facets, and answering to the tibia, fibula, and an additional element. The Liassic Plesiosaurian femur has only two distal facets, and but two principal bones in the second segment of the paddle. In some species, however (P. rugosus), a rudimentary third ossicle is present at the postaxial border of the fibula. The two principal bones, the tibia and fibula, in these latter Plesiosaurs are elongated in the direction of the axis of the limb, and they are very different in form from the tarsal bones ; whereas in the Kimmeridge Plesiosaurus the three coequal bones in the enemion are lengthened transversely to the axis of the limb, and differ little in shape from the tarsals. In this formal indifferentism of its enemion and tarsus, Mr. Mansel's Plesiosaurus has a resemblance to Ichthyosaurus ; but the similarity is more than balanced by the third bone in its enemion and the perfect formal difference of the digits.

It is, however, in the subgenus Pliosaurus that the limbs most