North American Species of Petromyzontidce. 289 



Lake lampreys (heads), 3 with 7 cusps. 



3 " 8 " 



4 " 9 " 



10 



No. with No. with No. with 

 Cayuga Lake Specimens. 7 cusps. 8 cusps. 9 cusps. 



Specimens with well developed ridge, 26 23 15 



Specimens without well developed ridge, 11 21 10 



Total, ...... 37 44 25 



Grand total, ... 106 



When there are more than seven cusps, usually from two to four 

 or five are smaller than the others. 1 noticed no variation in 

 the disposition of the other teeth. 



The mandibulary plate is proportionally about the same length 

 in all specimens ; its cusps are from 7 to 9. 



The caudal margin of the cephalic dorsal fin is always distinct. 

 In a few specimens the dorsal fins are well separated, the dis- 

 tance between them in some cases equals the distance from first 

 to fifth branchial fissure. This separation is greater and occurs 

 oftener in specimens without the dorsal ridge. 



The height of dorsal fins varies in different individuals. The 

 fins are usually lower when the distance between them is greater. 



The caudal margin of the caudal dorsal is in all specimens 

 quite distinct, the distance from its caudal margin to tip of cau- 

 dal varies in different individuals ; it is longer in the more slen- 

 der specimens. The distance from caudal margin of caudal 

 dorsal to tip of caudal fin is in some individuals nearly equal to 

 distance from caudal border of eye to seventh branchial fissure. 



A transection at nostril would cut or pass slightly cephalad of 

 cephalic margin of orbit. 



The distance from first to fifth branchial fissure is in some 

 specimens slightly greater than the distance from nostril to tip 

 of snout ; in some specimens equal to this distance, in others 

 much less. These characters vary as stated in all mature speci- 

 mens, whether large or small. Among the many larval speci- 

 mens examined, those of the same size all look alike. 



