INTEODUCTOEY COLLECTION. 
25 
into that of the single-fingered Horse, and another series ends with 
the hand of the ruminants, with only two fingers remaining, some- 
times, but erroneously, called a " cloven hoof." Similar changes 
are shown in the toes of the hinder extremity, illustrating the 
same common plan running through infinite modifications in 
detail, enabling the organ to perform such a variety of purposes, 
and to exhibit such diversity of outward appearance. The 
existence of this common plan is now generally thought to be 
due to inheritance from a common ancestor. 
In the lower part of the case are examples of all the different 
kinds of articulations or joints by which the various bones of the 
body are connected together. 
The central case of the bay contains a collection illustrat- Teeth of 
ing the principal characters of the teeth in the Mammalia. Mammals. 
Its inspection should commence at the north-east corner, where 
the visitor will find himself after completing the survey of the 
osteological specimens in the wall-cases. In the first division 
specimens are placed showing the general characters of 
teeth, their form, the different tissues of which they are 
composed, the two great types of dentition in mammals, 
liomodont and lietewdont, the nomenclature and homologies of the 
different teeth, and their development and succession. The 
principal modifications of teeth according to their function are 
next shown by examples of forms adapted for fish-eating, flesh- 
eating, insect-eating, grass-eating, &c. The remainder of the 
case is taken up by select examples of the dentition of all the 
different families of mammals arranged in systematic order, 
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. 
Bay ISTo. II. contains, in its first wall-case (the only one yet Bay II. 
occupied), illustrations of the outer covering or integument and ^f^^^ls 
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 para- 
chutes of flying animals. 
2. The development of true bony plates in the skin, found 
among mammals only in the Armadillos and their allies. The 
