COMPARATIVE ANATOMY 45 



they occur not only on the jaws, but may sometimes be found 

 on the vomer, palatine, pterygoid, and parasphenoid bones. 



Giant Salamander. — Compare with the fish. The teeth 

 are pleurodont, that is, attached to the inner surface of the jaw. 

 A similar condition exists in frogs, but our common toads have 

 no teeth. 



Turtle. — Adult turtles never possess teeth although rudi- 

 ments may exist in the embryo. The jaws are covered instead 

 by a thick hard horny layer, usually with a very sharp edge. 



Alligator. — The teeth have roots embedded in sockets in 

 the jaw (thecodont). This is unusual in reptiles, but is the 

 common condition in mammals. 



Bird. — Compare with the turtle. No living birds have 

 teeth, but some fossil forms were abundantly supplied with 

 them. 



Cat. — The teeth are thecodo?it (set in sockets), heterodofit 

 (of various shapes), and diphyodont ("milk" teeth precede the 

 permanent set). These conditions are typical of mammals. 

 The different kinds of teeth found in the cat have already been 

 studied; review them again if necessary. 



Rodent. — The long curved incisors, suited for gnawing, bear 

 hard enamel on the anterior face only. These teeth grow 

 throughout the life of the animal. 



Dental Formula. — The number and character of the teeth 

 are constant for members of the same species, but vary greatly 

 in different species. To express in a brief form the dental con- 

 dition, a dental formula is used. The formula for the cat is 

 I 3/3, C i/i, P 3/2, M i/i ; or omitting the letters, as is usually 

 done, the formula will read 



3, I, 3, I 



3, h 2, I 



Digestive and Respiratory Organs of the Shark 



If your shark is a female ask for instructions before opening 

 the body cavity; if you have a male open the body cavity by 

 making a longitudinal incision on each side from the pelvic 

 region to the pectoral, and then making transverse cuts at each 

 end so as to remove the ventral body wall. Most of the viscera 



