I am highly gratified in being able to lay before the Society an account of an almost perfect skeleton of Plesiosaurus*, a new fossil genus, which, from the consideration of several fragments found only in a disjointed state, I felt myself authorized to propound in the year 1821, and which I described in the Geological Transactions for that and the following year. It is through the kind liberality of its possessor, the Duke of Buckingham, that this specimen has been placed for a time at the disposal of my friend Professor Buckland for the purpose of scientific investigation. At the period of my former communications it was natural and even just that in the minds of many persons interested in such researches, much hesitation should be felt in admitting the conclusions of an observer who was avowedly inexperienced in comparative anatomy; and there might have then appeared reasonable ground for the suspicion that, like the painter in Horace, I had been led to constitute a fictitious animal from the juxtaposition of incongruous members, referable in truth to different species. But the magnificent specimen recently discovered at Lyme has confirmed the justice of my former conclusions in every essential point connected with the organization of the skeleton. The only material error which I have to correct relates to the bones which I supposed to be the radius and ulna: but with regard to the other parts of the skeleton, in assigning to the same animal the heads and vertebrae which