-508 



ANATOMY OF VERTEBRATES. 



The upper incisors are small, simple, and wide apart in the 

 upper jaw, 1 ; the anterior incisor in the lower jaw, 2, is long and 

 procumbent, but relatively smaller than in the Shrews ; the 

 canines are small in both jaws ; the premolars, 2, 3, 4, increase 

 in size and complexity as they approach the true molars, i, 2, 3. 



In Gymnura each premaxillary bone contains three teeth ; the 

 next has the form and size of a canine in both upper and lower 

 jaws, but has two roots in the upper jaw ; this is followed by four 

 premolars, the last of which, in the upper jaw, is large and quad- 

 ricuspid : the first and second of the true molars have square 

 multicuspid crowns; the last molar is smaller and triangular. 

 In the lower jaw the fourth premolar has a compressed tricuspid 

 crown. The dental formula of Gymnura is typical, viz. : 



.3.3 



1.1 



4.4 



.3 



The dentition of our common Hedgehog (Erinaceus europceus) 

 shows greater inequality in the upper and lower jaws, the formula 



beino; 



O 



3.3 



1.1 



l ; c - 

 3.3' 0.( 



3.3 

 2.2 ; 



m H = 36 ' fig '' 244 ' 



The first incisor in both upper and lower jaws is larger and 



244 2 longer than the rest, and 



is very deeply implanted 

 in the jaw ; the tooth 

 which follows the incisors 

 is small in both jaws, 

 but especially so in the 

 lower ; it may be called 

 a canine with two roots 

 in the upper jaw, p i. 

 The last premolar is the 

 largest in both jaws ; 

 above it has a quadri- 

 cuspid crown with three 

 fangs ; below, a subcom- 

 pressed tricuspid crown 

 with two fangs. The true 

 molars decrease in size 

 from the first to the third 

 in both jaws, the first 

 and second have sub- 

 quadrate four-pointed crowns above ; below, they are narrower, 

 and the anterior and inner angle is produced into a fifth cusp. 



Dentition of Hedgehog. 



1. Lower jaw, b side view, a working surface. 



2. Upper jaw, a working surface, b side view. 



