294 POPULAR HISTORY 01' THE AQUARIUM. 



fish it is a sac communicating only with the external sur- 

 face ; in every reptile it is a canal with both an external and 

 an internal opening. According to this test the Lepidosiren 

 is a fish.^^ The only opening of the nostril is within the 

 mouth. 



Onr Illustrations of Fishes are, Plate XV., Platessa vul- 

 garis, young, in attitudes described in the text ; and Plate 

 XVI., Lepidosiren swimming, wdth the bibernaculum shown 

 at the bottom. 



