622 ORDER CHONDROPTERYGII. 



tranche dombey, Lac. I. xxiii. 1. Petromyzon cirrhatus, 

 Forster, Bl. Schn. p. 532 \ 

 In 



Gastrobranchus, Block., 



The intervals of the gills, instead of having each 

 its particular issue externally, open into a common 

 canal for each side, and the two canals end at two 

 holes, situated under the heart, towards the first 

 third of the total length. 



But one is known, of the North Sea, Myxine glu- 

 tinosa, Linn., Gastrobranchus ccecus, Bl. 413. 



Ammoccetes, Bum., 



Have all the parts which should constitute their ske- 

 leton so soft and membranous, that they might be 

 considered as having no bone whatever. Their 

 general form and the exterior holes of the gills are 

 the same as in the Lampreys, but their fleshy lip is 

 only semi-circular, and covers only the upper part of 

 the mouth ; accordingly, they cannot fasten them- 

 selves to bodies, like the Lampreys, properly so 

 called. It is impossible to perceive any teeth, but 

 the aperture of their mouth is furnished with a range 

 of little branchial barbels. They have no particular 

 trachea, and their gills receive the water through the 

 oesophagus, as usual. Their dorsals are united toge- 

 ther, and to the caudal, in the form of a low and 



1 See the Article of Sir Everard Home in the Phil. Trans, of 

 1815. 



