9. Area directly before the mouth (within the marginal ring of teeth) 



toothless, with two small teeth on each side of this area; with 50 to 62 



muscle segments from the gills to the anus; Ohio and Potomac Valleys 



OWelhergia lamotteni (LeSueur) Ohio Lamprey 



Lampetra aepyptera (Abbott) 



Area directly before the mouth toothed; with 57 to 70 muscle segments 



from the gills to the anus; western North America 10. 



10. Dorsal fins well separated; parasitic 



Lampetra ayresii (Giinther) Western Brook Lamprey 

 Lampetra fluviatihs (Linn.) 

 Bases of dorsal fins connected; not parasitic 



Lampetra planeri Bloch Western Brook Lamprey 



11. Tail heterocercal (backbone ending above the center of the root of the 



tail fin or extending into the upper portion of the tail) ; large fishes 12. 



Tail homocercal (backbone ending at the center of the root of the tail 



fin) ^ 24. 



12. No scales or with rows of large bony plates 13. 

 With regular scales 20. 



13. No plates or shields: with two very tiny barbels beneath the paddlclike 



snout; Mississippi River; Family Polyodontidae 



Polyodon spathula (Walbaum) Paddlefish, Spoonbill 

 With a series of large bony plates, except in old fishes; with four distinct 

 barbels on the lower side of the upper jaw before the mouth; Family 

 Acipenseridae Sturgeons 14. 



14. No spiracular opening; caudal peduncle almost cylindrical, without 



plates 15. 



With a small spiracular opening before and above each opercular open' 



ing; caudal peduncle flattened, covered with bony plates 16. 



15. Belly with subrhombic plates: with 28 or 29 rays in the dorsal fin; Mis' 



sissippi drainage 



Scaphxrh.ynch.us platorynchus (Raf.) Shovel-nosed Sturgeon 

 Belly without plates; with 35 to 43 rays in the dorsal fin; Mississippi 

 and Missouri rivers; rare 



Parascaphvrhynchus alhns F. 6? R. White Sturgeon 



16. Dorsal fin with more than forty rays 17. 

 Dorsal fin with less than forty rays 18. 



17. With about forty plates along each side of the body; Pacific slope 



Acxpenser transmontanus Richardson Pacific White Sturgeon 

 With about thirty plates along each side of the body; ascending streams 

 from the Atlantic 



Acxpenser hrevirostris LeSueur Short-nosed Sturgeon 



18. With very many tiny plates between the dorsal plates and the row of 



enlarged lateral plates; Mississippi Valley; Great Lakes 

 Acipenser fulvescens Raf. Lake Sturgeon 

 {Acipenser ruhicundiis LeSueur) 

 With five to ten rows of small plates so situated 19. 



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