FEB., 1912. MAMMALS 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, f 

 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 teeth | 

 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. 



t The Monotremes furnish an exception, the teeth of the Duck-bill being of 

 horny construction. 



I 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. 



