CLASSIFICATION OF VERTEBRATA 



II. CLASS PETKOMYZONTES 

 = HYPEKOAKTIA, Mueller 



Nasal sac not communicating with the mouth-cavity. 

 The gill-pouches combine internally into one canal, which 

 is situated below the oesophagus, and opens into the mouth- 

 cavity. 



Ear with two semicircular canals. Cosmopolitan. 



Petromyzon, Linne. In Europe : P. marinus, P. fluviatilis, 

 P. planeri. 



Here probably also : 



Palaeospondylus gunni. Traquair, 1890. Old Eed Sand- 

 stone of Caithness. 



