LAMPREYS AND HAG-FISHES. 



553 



morphosis in salt water. Always restricted to low-lying countries, this lamprey 

 may be found alike in rivers, streams, lakes, and marshes, although it only spawns 

 where the water is clear and flows swiftly over a stony bed. During the spawning- 

 season, which takes place in March and April, the lampreys acquire a brilliant 

 metallic lustre ; while at the conclusion of the function they generally perish. 

 Formerly these lampreys occurred in enormous quantities in many of the English 

 rivers, upwards of three thousand having been taken at Newark in a single night ; 

 but the numbers in the Thames are now considerably diminished. Their chief use 

 is as bait for cod and other fish ; for which they are specially adapted on account 

 of the ease with which they can be kept alive. There is nothing calling for special 

 notice with regard to the habits of the small lamprey. 



southern In the Southern Hemisphere the family is represented by three 



Lampreys, genera, in one of which there is a single species (Mordacia mordax) 



common to the coasts of Chili and Tasmania ; while in a second (Geotria) there is 



one Chilian and another South Australian species. The first of these two genera 



agrees with the typical representatives of the family in the continuity between 



■U-^ 



hag-fish (g nat. size). 



the second dorsal and caudal fins, but differs in having two groups of three-cusped 

 teeth above the aperture of the mouth ; whereas in the second genus the two fins 

 above-mentioned are separate, and there is a four-lobed plate above the mouth. 

 Some of these lampreys grow to a length of a couple of feet ; and in the adults of 

 some or all of them the skin of the throat is so much expanded as to form a kind 

 of pouch. The third genus (Exomegas) appears to be known only by two 

 examples from the Atlantic side of South America, one of which was picked up 

 in the streets of Buenos Aires in 1867, while the second was obtained from the 

 Bay of Monte Video in 1890. With the exception that the dentition is of a 

 peculiar type, very little is known as to the structure of this rare form. It will 

 not fail to be noticed that the remarkable geographical distribution of these 

 southern lampreys is paralleled by that of certain fresh-water fishes already 

 described, with the exception that there is no instance among the latter where 

 a species is common to Australia and South America. 



The hag-fishes, of which there are two genera, constitute a 

 family (Myxinidce) distinguished from the last by the nasal sac 

 having a posterior duct which perforates the palate ; the single external nasal 



Hag-Fishes. 



