PETROMYZONTS OR LAMPREYS 453 



spends its larval life in fresh water. This large lamprey is 

 found along the coasts of Europe and North America, and is 

 represented in the interior waters of New York by a land-locked 

 variety. 



Ichthyomyzon. — Girard. Small lampreys, confined to the 

 rivers of the Mississippi Valley and eastern United States. The 

 supraoral plate is typically armed with two or three (sometimes 

 four) separate teeth, set close together; the anterior lingual 

 tooth has a median groove ; the dorsal fin is continuous with a 

 broad and shallow notch. Example: /. concolor (Kirtland), 

 the silvery lamprey, ten inches long. 



Lampetra. — Small lampreys of the streams of Europe and 

 North America. The supraoral plate is crescent-shaped, with 

 a large bluntish cusp at each end, separated by a very small 

 median cusp ; the lingual teeth are small, with dentate edges, 

 the median denticle enlarged ; the dorsal fin has a sharp notch 

 or is entirely divided. Examples : Lampetra planeri, the 

 lesser fresh-water lampern of Europe, usually under a foot in 

 length, and the North American L. wilderi, 6 to 10 in. in length, 

 apparently not attaching itself to fishes. 



Besides these genera there are others : e.g., Mordacea on 

 the coasts of Chili, Australia, Tasmania ; Geotria on the coasts 

 of Chili, Australia, New Zealand. 



Affinities of Cyclostomes 



(1) It must be admitted that Cyclostomes and fishes have 

 several features in common besides the essential vertebrate 

 characters. In both classes, for instance, there is a two- 

 chambered heart which drives the blood by a ventral aorta to 

 the gills. 



(2) But if the word " fish " is to mean anything precise, it 

 cannot include Cyclostomes, if only because they are jaw- 

 less. 



(3) That they are not degenerate fishes, is plain when we 

 consider, for instance, the muscular rasping "tongue," the 

 peculiar naso-pituitary canal, and the saccular gills. 



(4) That the Cyclostomes probably diverged from the 

 vertebrate stem at a level far below that of fishes, is suggested 

 by many facts, such as the absence of jaws, the persistent un- 

 segmented notochord which is continuous in front with the 



