As the following pages contain merely the substance of two Lectures delivered by the Author before the Royal Agricultural Society on the 17th and 24th May in the present year, it might perhaps be thought that no prefatory observations were called for. He, however, owes it to his readers to explain as briefly as possible the circumstances that have led to their being given publicity to. In his official capacity as Veterinary Inspector to the Society, he was requested by the Council to offer some remarks on the structure and development of the Teeth, and the changes that take place in those organs by which the different ages of the Ox, Sheep, and Pig are indicated. The subject was not new to him, and long-continued and repeated observations had convinced him that the opinions advanced by veterinary writers, both of this country and France, were often incorrect. This, added to the importance of the question in the present day to agriculturists, made him doubly solicitous that the views he was thus about to promulgate should be founded on data both sufficiently numerous and correct. Exceptions will doubtless now and then be met with to the rules he ventured to lay down, arising from peculiar conditional circumstances under which the animals may have been placed; but these, he believes, will be found to be only few, and as such they will serve to make the laws general one. Above 2000 animals had been examined by him, whose ages were correctly known.