KEY TO GENERA OF PETROMYZONID^I FOUND IN ILLINOIS 



Key to Genera op PETROMYZONID^ found in Illinois 



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

 ous 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 Lampetra. 



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 gr6ove; 

 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) 



Kirtland, '40, Bost. J. Nat. Hist., Ill, 342 (Petromyzon argenteus) ; id., 1. c, 473 

 (Ammoccetes) ; Girard, '58, Pac. R. R. Surv., 381,<582 (castaneus and hirudo). 



G., VIII, 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 conspicuous 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 circum- 

 ference 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 (an- 

 terior) 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 unicuspid, though this character is subject to much variation, 



