The central case of the bay contains a collection illustrating the principal characters of the teeth of Mammals. Its inspection should commence at the north-east corner, where the visitor will find himself after completing the survey of the specimens of skeletons in the wall-cases. In the first division are placed specimens showing the general characters of teeth, their form, the different tissues of which they are composed, the two great types of dentition in Mammals, _homœodont_ and _heterodont_,[5] the names and serial correspondence of the different teeth, and their development and succession. The principal modifications of teeth according to function are next shown by examples of forms adapted for fish-eating, flesh-eating, insect-eating, grass-eating, etc. The remainder of the case is taken up by examples of the dentition of the families of Mammals arranged in order, and prepared so as to display not only the shape of the crowns, but also the number and character of the roots by which they are implanted. [Sidenote: Bay II. Classification of Mammals.] In bay No. II the two wall-cases contain a collection arranged to show in a serial manner the orders and sub-orders of existing Mammals, by examples selected to illustrate the predominating characters by which these are distinguished. A brief popular account of the characteristics of the group, and a map showing its geographical distribution, are placed with each. This is intended to serve not only for an introduction to the study of the class by visitors to the museum, but also as a guide to a method of arrangement which may be adopted in smaller institutions. Among the illustrations of the order _Primates_ is placed the skeleton of a young Chimpanzee dissected by Dr. Tyson, which formed the subject of his work on the “Anatomy of a Pigmie,” published in 1699, the earliest scientific description of any Man-like Ape. [Sidenote: Skin of Mammals.] The central case of this bay contains illustrations of the outer covering or skin and its modifications in the class of Mammals, divided into the following sections: 1. Expansion of skin to aid in locomotion, as the webs between the fingers of swimming and flying animals, the parachutes of flying animals. 2. The development of bony plates in the skin, found among Mammals only in the Armadillos and their allies. The cast of a section of the tail of a gigantic extinct species (_Glyptodon_) shows a bony external as well as an internal skeleton. 3. The outer covering modified into true scales, much resembling in structure the nails of the human hand. This occurs in only one family of Mammals, the Pangolins, or _Manidæ_. 4. Hair in various forms, including bristles and spines. The two kinds of hair composing the external clothing of most Mammals, the long, stiffer outer hair, and the short, soft under-fur, are shown by various examples.