74 



ZOOLOGY. 



Besides the true Gulls (Larus), I will only mention 

 the Sea Swallows (Sterna), with forked tails. 



Fig. 4d. — Herring Gull (^Larus argentatus). 



CLASS III. : REPTILIA (REPTILES). 



Cold-blooded Vertebrates (p. 19). The heart has 

 the same structure as in Mammals and Birds, but the 

 left and right ventricles are usually only incompletely 

 divided from one another (p. 20). The body is invested 

 with horny scales or with bony plates covered by a 

 horny layer. Reptiles are very unlike Birds in appear- 

 ance, but present many essential points of resemblance 

 to them in their skeleton ; indeed, during earlier geo- 

 logical times, there were transitional forms between 

 the two classes. The structure of the reptile egg, too, 

 is very similar to that of the bird's egg ; but the first 

 has no lime-salts deposited in its shell. The egg is 

 not hatched out by the animal itself, but exposed to 



