Unhcrsity of Michigan 



f^. Anterior disc teeth very numerous and crowded, in about 

 six rows medially; enlarged laterals four on each side, 

 the first and last bicuspid, the median two either bicuspid 

 or tricuspid ; anterior lingual lamina not at all bent inward 

 medially, the edge comb-like with long, pointed serrations; 

 supraoral often with one or two small denticles located 

 laterally on the bridge between the main cusps; infraoral 

 with nine teeth, alternately larger and smaller. Rio Lerma 

 basin. Mexico. Entosphenus {Tetraplenrodon)''' spadiceus. 



i~. Anterior disc teeth not very numerous or crowded ; 

 enlarged laterals normally four in number, the first and 

 last bicuspid, the median two tricuspid (one of the median 

 two rarely lacking, a fifth occasionally added by the doub- 

 ling of the first tooth of the posterior series) ; anterior 

 lingual lamina slightly bent inward medially, the edge 

 dentate, the median cusp little enlarged; supraoral always 

 tricuspid, the third cusp median ; infraoral cusps 4 to 6, 

 uniform in size, the outermost not doubled. 

 Subgenus Entosphenus 



gi. Myotomes between the last gill opening and vent 68 

 to 74. Pacific Ocean and coastal streams from Una- 

 laska to the Columbia River System. 



Entosphenus tridentatus tridentatus. 



g-. Myotomes between the last gill opening and the vent 

 57 to 67. Pacific Ocean and coastal streams from 

 southern Oregon to southern California. 



Entosphenus tridentatus cUiafus. 



P. Anterior disc teeth not very numerous or crowded ; 

 enlarged laterals always three in number, all bicuspid ; 

 anterior lingual lamina not at all bent inward medially, 

 the edge dentate or crenulate, with the median teeth 

 notably enlarged ; supraoral rarely with a median or sub- 

 median denticle, at most weakly developed; infraoral cusps 

 8 to II, the outermost two of each side imperfectly sep- 

 arated. 



Subgenus Lcthentcron'^ 



^ Tetrapleurodon Creaser and Hubbs, new subgenus ; orthotype, 

 Lampetra spadicea Bean. 



"' Lethenteron Creaser and Hubbs, new subgenus: orthotype, Lam- 

 petra wilderi Gage (= Petro7nycon appendix DeKay). 



