XXXVII.— A Description of a Specimen of the Plesiosaurus Macrocephalus, Conybeare , in the Collection of Viscount Cole, M.P., D.C.L., F.G.S., &c.

Plates XLIII., XLIV., XLV. Lord cole having done me the honour to transmit to me for description, his unique specimen of the Plesiosaurus macrocephalus , I have endeavoured to devote to it an examination equivalent in care and detail to the admirable completeness of many of its parts, and have reason to hope that it will throw additional light on the Plesiosaurian modifications of the vertebrate skeleton, as well as on some interesting points in general osteology. The study of the spinal column has induced me to reconsider the views generally adopted as to the composition of a vertebra; and I have found it convenient, if not absolutely requisite, to give new names, applicable both in the abstract and concrete senses, to some of the component vertebral elements. The chief specific characters are also premised of another Plesiosaur, which, from the completeness of its skeletons in our national and other collections, it seemed most advantageous to adopt as a term of comparison with the present specimen. The species to which I refer, is that which is described and figured in Mr. Hawkins’s memoir on Ichthyosauri and Plesiosauri *, under the name of Triatarsostinus ; but as this designation relates to an imperfect state of the tarsus in the right foot, (for a fourth bone is present in the left tarsus of the same specimen, and a second specimen of the same species in Mr. Hawkins’s collection exhibits five tarsal bones on each side,) I propose to refer to it under the name