The following research is a continuation of that previously communicated to the Society on the “Minute Anatomy of the Thyroid Gland of the Dog," and published in the Philosophical Transactions for 1876 (Vol. 166, Part II.). It is based on a histological examination of the thyroid gland in the following vertebrate animals:— I. Mammalia . Dog, Cat, Horse, Rabbit, Ox, Sheep, Seal. II. Aves . Pigeon, Fowl, Book. III. Reptilia . Tortoise. IV. Amphibia . Frog. V. Pisces . Skate, Conger Eel. (Altogether the glands of more than sixty animals have been examined in this research.) My examination of the human thyroid gland is unfortunately not sufficiently complete to allow of its being included in this communication, partly owing to its great delicacy of structure, and partly to the difficulty of obtaining this gland perfectly fresh, a condition which is absolutely necessary in order to examine its minute anatomy. I shall therefore merely refer to this gland incidentally on one or two points in the course of the paper.