FISHES — PETEOMYZONTIDAE — ICnTHYOMYZON. 



381 



margin of the buccal orifice, which exhibits six nearly equal teeth, in which respects it resem- 

 bles more P. plumhetis. The eyes are large, sub-circular, and rather inconspicuous. The base 

 of the second dorsal fin enters seven times in the total length, being, therefore, exactly an inch 

 long. The second dorsal passes to the upper lobe of the caudal fin by a shallow and gradual 

 depression. There is no trace of anal fin, and the lower lobe of the caudal is more extended, 

 and posteriorly deeper than the upper lobe. The caudal itself is lanceolated, tapering into a 

 point. The vent is placed somewhat posteriorly to the origin of the second dorsal fin. 



The color is yellowish brown, of a deeper tint along the dorsal region and upper surface of 

 the head than beneath. 



List of specimens. 



ICHTHYOMYZON, Girard. 



Gex. Char. — Buccal disk funnel-shaped, sub-terminal, inferior, sub-circular or sub-elliptical, fringed upon its periphery. 

 Two vertical, finely denticulated jaw pieces, one on either side of the esophagus ; a similar horizontal piece below the 

 former. Teeth within the buccal disk generally simple, curved, disposed upon lateral cgees, largest inwardly. Gills in 

 separate partitions within the chest. One single dorsal fin united to the upper lobe of the caudal. No anal fin. 



This genus differs from Petromyzon by the form and disposition of the teeth, and by the 

 presence of one dorsal fin instead of two. The horizontal jaw piece has the shape of two arcs of 

 a circle soldered together, whilst in Petromyzon the same cartilaginous piece is nearly transverse. 

 A rather large, tricuspid tooth, may be observed at the anterior margin of the buccal aperture, 

 instead of a transverse sub-crescentic piece, armed with three distinct hooks ; and at the posterior 

 margin of the same aperture is a series of uniform teeth, closely set together upon an arc of a 

 circle, instead of the five distant unequal teeth as observed in Petromyzon. 



To this genus belong: P. planeri, Bl. oC Eu;ope; and P. lamottenii, liESv; P. argenteus, 

 Kirtl; p. appendix, DeKay ; and the following two species. 



1. ICHTHYOMYZON CASTANEUS, Grd. 



Spec. Chae. — Head depressed, constituting the ninth of the total length; body and tail compressed. Buccal disk sub- 

 elliptical, provided with a double series of short, tentacular fringes upon its periphery. Posterior margin of buccal aperture 

 exhibiting a series of nine teeth, disposed up m an arc of a circle. Eyes sm ill and inconspicuous. Spiracle sub-tubular, raised 

 above the surface of the head. Origin of the dorsal fia equidistant between the an erior margin of the buccal di^k and the 

 apex of the tail. Vent situated immediately in advance of the most elevated portion of the dorsal fin. Chesnut colored, of 

 a darker tint above than beneath. 



What we have termed head is measured from the anterior extremity of the buccal disk to the 

 first branchial orifice, the chest being the region occupied by the entire series, seven in number, 

 of the same branchial orifices. 



The length of the head is equal to that of the chest. The tentacles, at the periphery of the 

 buccal disk, are inserted into a shallow groove, formed exteriorly by the thickened edge of the 

 disk, and interiorly by a soft and flexible membranous ridge. The fringes themselves are more 

 developed posteriorly than anteriorly. The branchial orifices are sub-ciroular, provided with 



