( Ichthyomyzon fossor )^ the silver lamprey (!» unicuspis), and 

 the chestnut lamprey (I. castaneus ) . The larvae of each of these 

 various species^ native and introduced^ are notoriously difficult 

 to distinguish from one another. As yet, no artificial key based 

 on taxonomic characters has been presented ■which would aid in 

 differentiating all forms present fro^r: each other « 



Certain anatomical features characteristic of the adults of 

 several species are distinguishable in the larval stage and iden- 

 tify the ammocoetes of these species at least to genera. Adult 

 members of the genus Ichthyomyzon possess a single^ continuous 

 dorsal fin -which is recognizable even among its vei^" small larvae » 

 Adult lampreys of the genera Entosphenus and Petromyzon have two 

 distinct dorsal fins which are likeivise present among their larvae o 

 Vladykov (19ii9) has demonstrated that the number of trunk myomeres 

 of the silver lamprey and Michigan brook lamprey (genus Ichthyomyzon ) 

 is less than in the general Entosphenus and Petromyzon . The ranges 

 in number of trunk myomeres of the latter two genera overlap broad- 

 ly and render identifications between them based on this feature 

 impractical. 



Utilizing the two features just described^ i|2 ammocoetes be- 

 longing to the genus Ichthyomyzon were sorted out of the collee-- 

 tions. Two transforming silver lampreys vrere also removed. It is 

 unlikely that any larvae of the chestnut lamprey were among those 

 thus removed as the Ocqueoc River is outside the natural range of 

 this species in Michigan » 



Presumably larvae of only two species, the sea lamprey and 

 the American brook lamprey remained in the collections o No effort 

 was made to distinguish betT/een the larvae of these genera,, Ad- 

 mittedly, then^ some infusion of Entosphenus larvae is present in 

 my series of ammocoetes but I believe the number of these must be 

 negligible,, During three years of work in the Ocqueoc River water- 

 shed in which I became intimately acquainted with all lamprey 

 spawning areas in the river, I have never seen either spawning or 

 spent American brook lampreys. Among collections made in all types 

 of habitats in all seasons of the year^ only one transforming 

 specimen of the non-parasitic American brook lamprey was ever talcen. 

 Obviously the species is sparse in the Ocqueoc watershed. On the 

 other hand, spawning sea lampreys are annually abundant to the 

 point of severe overcrowding in some spawning areas. Enormous 

 numbers of their offspring appear in the silted and sandy areas of 

 the river after each spawning season,. 



It may be concluded that my collections are almost entirely 

 composed of ammocoetes of the sea lamprey and that the numbers of 

 American brook lamprey larvae in the series are so small^ if pre- 

 sent at all, that they could not invalidate the analyses and con- 

 clusions subsequently drawn therefromo 



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