CYCLOSTOMATA. 293 



tains a mucilaginous matter. There are no ordinary ribs, but the branchial ribs 

 are much developed, forming a sort of chest for containing the gills, which, in place 

 of having the usual appearance of leaves, or combs, are united each to the adjoin- 

 ing one so as to form oval bags. The nostrils open by a single orifice, in front of 

 which there is the entrance to a blind cavity, that has been erroneously considered 

 as an air-hole or spiracle. The gut, slender and straight, is furnished with a spiral 

 valve. 



[124.] 1. Petromyzon trident atus. (Gairdner.) Tridentate 



Lamprey. 



Family, Cyclostomafa (Les sticeurs). Cuv. Genus, Petromyzon. Ccv., Linn. 

 Petromyzon tridentatus. Gairdner, in lit. 

 Squaqual. Indians of the banks of the Walamet. 



The lampreys have seven gill-openings on each side ; their maxillary ring is 

 armed with strong teeth, and the inner surface of the circular lip is likewise 

 provided with tubercles encrusted by a hard shell and resembling teeth. The 

 tongue, furnished with two longitudinal rows of little teeth, works backwards and 

 forwards like the sucker of a pump. The water passes from the mouth to the gills 

 by a membranous tube situated under the oesophagus and pierced by lateral holes. 

 There is one dorsal fin anterior to the anus, and another behind it which unites 

 with the caudal ; the latter looks like a mere fold of the skin, its rays being fine 

 fibres that are scarcely perceptible. These fishes adhere strongly to stones and 

 other bodies by their mouth, and also to fishes, the largest of which they can destroy 

 by draining them of their fluids. Three species are described as European, viz., 

 the P. marinus, fluvialis, and planeri, the two former of which are mentioned by 

 Mitchill and Smith as inhabitants of the United States waters. 



The tridentate lamprey abounds at the falls of the Walamet, which we have 

 already had occasion to say is one of the tributaries of the Columbia. Dr. Gairdner 

 gave it the name of tridentatus, from the upper side of its maxillary ring being 

 armed with three conspicuous and contiguous teeth, of which the middle one is the 

 smallest. It resembles P. marinus and Jluvialis in the dorsals being distinct. 



DESCRIPTION. 



Mouth broadly oval, its axis parallel to that of the body. The thick obtuse lips are furnished 

 with a circular row of small nipple-like papilla?, about fifty-six in number, each standing in the 



