FrEB., 1912. MamMAts oF ILLINOIS AND WISCONSIN — Cory. 13 
The Skull — The skull is composed of a number of bones which, with 
the exception of the lower jaw, are immovably joined together. 
In early life they are largely connected by intervening tissue, which 
in most cases becomes ossified later. The “‘cracks”’ seen in a skull 
where the bones join are called sutures.* Holes and openings 
are called foramina; and elongated points (processes), ridges, cavi- 
ties, etc., are all recognized by names (see illustrations, pages 11-12). 
The Teeth — Teeth are hard formations which are present in the jaws 
of most mammals. They are enderonic structures, which develop 
from odontoblasts and are chiefly composed of calcium phosphate.t 
The greater portion of a tooth is made up of what is called dentine, 
having a cavity in the center containing what is known as “pulp,” 
a soft mass of connective tissue supplied with blood vessels and 
nerves. The exposed outer surface of the tooth is covered with a 
thin but extremely hard layer known as the enamel. From a sys- 
tematic standpoint the teeth are of great importance, inasmuch as 
their various well defined characters furnish differences upon which 
many of the genera and species, as well as some of the higher orders, 
are based. The majority of mammals have two sets of teetht 
known respectively as ‘‘milk teeth’? and permanent teeth. The 
former are the temporary teeth of the young and are later replaced 
by permanent teeth. In mammals which have both milk and 
permanent teeth, the number of the former varies greatly in different 
species, ranging from a single tooth on the side of each jaw (as in 
the Marsupials and some Rodents) to the greater portion of the 
series. In some cases milk teeth are present but are not functional, 
and do not appear above the gum. Mammals in which the first 
teeth are permanent are called Monophyodont, while those which 
have in early life desiduous teeth, which are later replaced by 
permanent teeth, are called Diphyodont. 
Teeth are divided into four groups which may be described as 
follows: 
Incisors: Teeth with cutting edges and simple roots, which are 
implanted in the premaxillary bone in the center or front of 
the jaw. 
Canines: Usually four in number, two in the upper jaw and two 
in the lower.§ They are situated (when present) in the maxil- 
* For further remarks on sutures, see page 37. 
+ The Monotremes furnish an exception, the teeth of the Duck-bill being of 
horny construction. 
t{ The Monotremes, Toothed Whales and most of the Edentates have but one. 
§The Shrews apparently have two canine teeth in the upper jaw, but none in 
the lower. 
