CLASSIFICATION OF VERTEBRATA 



II. CLASS PETROMYZONTES 

 = HYPEROARTIA, 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, 

 I'. planeri. 



Here probably also : — 



Palaeospondylus gunni. Traquair, 1890. Old Red Sand- 

 stone of Caithness. 



