LAMPREYS AND HAGS 61 
the pharynx; this, with other related conditions, has caused 
Myxine and Bdellostoma to be included in a sub-group 
of Cyclostomes, as Myxinoids, or Hyperotretes.* In each 
genus there is possibly no more than a single valid species. 
Myxine is a well-known form: it occurs along the Atlan- 
tic coast at moderate depths. It is exclusively carnivorous, 
Fig. 72.— A-D. Ventral aspects of headg of (A) p 
Bdellostoma (after AYERS); (2) Myxine (after GUN- 
THER); (C) Ammocetes (after GUNTHER) ; (D) Pe- 
_fromyzon (after GUNTHER). 
often boring its way into the abdominal 
cavity of (diseased or injured) fishes, and 
with them is brought to market; it is 
also taken not infrequently by line fisher- 
men. The smallest example that has 
thus far been described is 6 cm. in length; it was 
recorded by Beard. (V. Réf. p. 239). 
The Lamprey, Petromyzon, is the most perfectly studied 
member of the Cyclostomes. Its species are common 
to the continents of the northern hemisphere; and in 
South America and Australia there occur very closely 
allied genera, as Mordacia and Geotria. The largest 
lamprey, P. marinus (Fig. 72, and C, D), is known to 
attain a length of nearly four feet ; it occurs in the coast 
* v. Glossary, p. 228. 
