K IV TO GENERA OF rETROMYZONIDJE FOUND IN ILLINOIS 



Key to Genera of PETROMYZONID^ found in Illinois 



a. Supraoral cusps 2 or 3 in number, placed close together; dorsal fin continuous, 

 with a broad notch Ichthyomyzon. 



aa. Supraoral cusps spaced wide apart, one at each end of a crescent-shaped 

 plate, which may bear a rudimentary median cusp; dorsal fin with a sharp 

 notch La mpetra. 



Genus ICHTHYOMYZON Girard 



river lampreys 



Supraoral plate typically armed with 2 or 3 (sometimes 4) separate 

 teeth, set close together; anterior lingual tooth with a median groove; dorsal 

 fin continuous, with a broad and shallow notch. Small lampreys, confined 

 to the rivers of the Mississippi Valley and eastern United States. 



ICHTHYOMYZON CONCOLOR (Kirtland) 



silvery lamprey 



(Pl., p. 16) 



Kirtland. MO, Bost. J. Nat. Hist., Ill, 342 (Petromyzon argenteus) ; id., 1. c, 473 

 < Ammoccetes) ; Girard, '58, Pac. R. R. Surv., 381, 382 (castaneus and hirudo). 



O..YIII. 507 (hirudo); J. and G., 10 (argenteus); M. V., 10 and 11 (Petromyzon 

 castaneus and concolor) ; J and E., I, 11 (castaneus and concolor) ; N., 52 



• argenteus and hirudo); J., 70 (Ammoccetes argenteus and hirudo); F., 86 

 (argenteus); L,., 7 (concolor and castaneus). 



Length 10 inches; depth 9.8 to 13.8 in length; width of body 1.4 to 2 in 

 its depth; distance from last gill-opening to front of dorsal fin 3.3 to 3.8 in 

 length; last gill-opening to vent 1.9 to 2.2; muscular impressions (between 

 last gill-opening and vent) 49 to 55. Color silvery, bluish above, sometimes 

 with bluish spots; a small dusky spot above each gill-opening, usually con- 

 spicuous even in the larva. Head (to first gill opening) 6.5 to 8.3 in length; 

 diameter of expanded buccal disk about % length of head, a double row of 

 fimbriae about the circumference of the disk, inside of which is a thin flexible 

 lip; eye 6 to 8 in head to first gill-opening; anterior lingual tooth with a median 

 (anterior) groove; supraorals typically bicuspid, occasionally with one, 

 three, or four cusps; infraorals typically 7 to 9, occasionally 10, and in one of 

 our specimens 13; extraorals, when supraorals are bicuspid, as a rule uni- 

 cuspid, though this character is subject to much variation, instances of as 

 many as 6 or 7 bicuspid extraorals having been noted in specimens with 

 bicuspid supraorals.* 



* A study of our 31 specimens of Ichthyomyzon shows an amount of intergradation in dental 

 characters that makes impossible the separation of the nominal species castaneus (=concolor), 

 as is evident from the following tabulation: 



—10 F 



