KEY TO GENERA OF PETROMTZONID^ FOUND IN ILLINOIS 



Key to Genera of PETROMYZONIDi<E found in Illinois 



a. Supra oral 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 Lampetra. 



Genus ICHTHYOMYZON Giraed 



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 (Kietk^nd) 



SILVERY LAMPREY 



(Pl.. p. 16) 



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

 ( Ammocoetes) ; Girard, '58, Pac. R. R. Surv., 381, 382 (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 (Ammocoetes 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 studj- 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 



