CYCLOSTOMES. 223 



peculiar structures known as conodonts were regarded as myx- 

 inoid teeth, but later these have been supposed to be annelid 

 jaws. Traquair has recently described, under the name Pal<zo- 

 spondyhcs gunni, a fossil from the Devonian of Scotland, which 

 may prove to be a palseozoic marsipobranch, or possibly the-, 

 larva of some higher form. 



SUB-CLASS I. PETROMYZONTES (HYPEROARTIA). 



Marsipobranchs with well-developed dorsal fins ; hypophysial 

 duct closed, its external opening on the top of the head ; seven 

 branchial openings on either side ; branchial basket well devel- 

 oped ; pharyngeal region divided by a longitudinal partition into 

 a dorsal food tube and a ventral respiratory duct from which the 

 gill slits arise ; a slightly developed spiral valve in the intestine. 



The lamprey eels live both in salt and in fresh water, some 

 of the marine species ascending rivers in the spring to lay their 



Fig. 228. See lamprey, Petromyzon marinus, after Goode. 



eggs. When ovipositing they attach themselves to stones, or 

 take up smaller pebbles with their suckers to make their nests,, 

 a fact which is reflected in the generic names Petromyzon and' 

 Lampetra. The lampreys feed upon the mucus and blood which, 

 they rasp from fishes to which they attach themselves. The 

 lampreys are included in a single family, Petromyzonid^, and 

 are grouped in several genera. 



Petromyzon, second dorsal joined to caudal, supraoral tooth with 2-3 

 cusps. P. marinus (sea lamprey), Europe and North America — Atlantic. 

 Lampetra, smaller species (brook lampreys), with broad supraoral tooth 

 with median cusp small or lacking. L. planeri, Europe. Scarcely different 

 is L. ivilderi from New York. Other genera are Mordacia and Geotrio- 

 from the southern hemisphere. 



