44 Memoir Sears Foundation for Marine Research 



Discussion of Genera. Generic characters among the Petromyzonidae, as here lim- 

 ited, are afforded by the dentition and by the number of dorsal fins, i.e., whether one or 

 two. Seven genera are recognized in the most recent general synopsis of the family as 

 limited above/' Five of these are known in the northern hemisphere, but only two, namely 

 Petromyzon and Lamfetra, occur in the North Atlantic. Petromyzon inhabits only the 

 western North Atlantic, while Lampetra, which also is marine and anadromous along the 

 coasts of Europe and northern Asia, is confined to fresh water in North America. 



Key to Genera of the Northern Hemisphere*" 

 la. Only i dorsal fin. Ichthyomyzon Girard, 1858. 



Eastern North America.*' 



lb. More than i dorsal fin. 



2a. Teeth on oral disc, sometimes called labial teeth, close together, arranged in 

 curvilinear radiating rows (Fig. 2D). 



3a. Supraoral dental plate with 2 large teeth; margin of anterior lingual dental 

 plate deeply indented in the midline. 



Petromyzon Linnaeus, 1758, p. 45. 



3b. Supraoral dental plate with only i tooth; margin of anterior lingual dental 



plate not deeply indented in the midline. Caspiomyzon Berg, 1906. 



Caspian Sea. 



2b. Teeth on oral disc loosely spaced, not in radiating rows. 



4a. Supraoral dental plate with a strongly developed sharp median tooth, as well 

 as 2 still larger lateral teeth on each side. Entosfhenus Gill, 1862. 



Pacific Coast of North Amer- 

 ica, from California to 

 Alaska."' *^ 



4b. Supraoral dental plate without strong, sharp, median tooth, at most with i 

 or more low, blunt, median denticles. Latnpetra Oken, 1 816.°° 



Both coasts of North Atlantic 

 and western Pacific."' 



45. Holly, in Schultze, Kukenthal, et al., Tierreich, Lief 59, 1933: 12. 



46. Somewhat amended from the synopsis by Berg- (Annu. Mus. 200I. Acad. Leningrad, 32 [i], 1931: 87). For 

 synopses of the family as a whole, see Regan (Ann. Mag. nat. Hist., [8] 7, 191 1 : 193) and Holly (in Schultze, 

 Kukenthal, et al., Tierreich, Lief 59, 1933; 13). 



47. In fresh water exclusively. 



48. Some of the species that fall in Lamfetra by this key are placed in Entosfhenus by Creaser and Hubbs (Occ. Pap. 

 Mus. Zool. Univ. Mich., 120, 1922: 6) ; if accepted, this would expand the range of the genus to northeastern 

 United States, Mexico and Alaska in fresh water; to Japan and the White Sea. 



49. Marine, but entering fresh water to breed. 



50. Including Eudontomyzon Regan, 191 1, which was classed as a subgenus of Petromyzon by Creaser and Hubbs 

 (Occ. Pap. Mus. Zool. Univ. Mich., 120, 1922: 2), as a subgenus of Lamfetra by Berg (Annu. Mus. Zool. Acad. 

 Leningrad, 32 [i], 1931: 92) and by Holly (in Schultze, Kukenthal, et. al., Tierreich, Lief 59, 1933: 22), 

 with whom we agree. 



51. Europe, northern Asia, Japan, North America, Mexico; some species confined to fresh water; others marine, 

 but entering fresh water to breed. 



