24 



ON THE LAMPHEYS OF JAPAN AND SIBERIA. 



differences which distinguish it from the latter. It must, therefore, be 

 regarded as a distinct species ; and I have great pleasure in calling it 

 Lampetra mitsukurii in the honour of Prof. Mitsukuri. 



The size varies within certain limits ; it is not infrequent that the 

 individuals from some localities are double those from others. 



The American Brook Lamprey, Lampetra wildert (Gage) U 2) is very 

 close to this species ; it is, as it seems to me, probably its nearest ally 2 ). 



I may perhaps be permitted to add a few words in regard to the brain 

 of two Japanese species of Lamprey. I find that the adult individuals 

 of the both species are remarkably alike in the external configurations of 

 their brain, which are again very close to those of P. fluviatili* of 

 Europe, (see Ahlborn's 4 ) figs 4 — 6). Therefore, if Ahlborn's statements 

 and figures (figs. 1 — 3) on P. planeri be trusted which maintain some 

 difference between its brain and that of P. fluviatilis, they might be taken 

 to be opposed to the idea of a near affinity between P. planeri and L. 

 mitsukurii mentioned above. But there is a strong reason doubting the 

 accuracy of Ahlborn's results. It appears to me that the brain of P. 

 planeri as given in his fig. 1 is that of a young individual, for the brain 

 obtained from the oldest Ammocoetes stage of the smaller Japanese 

 species shows very much the same configurations. This doubt is further 

 confirmed by the fact that the figures (figs. 5 & 6) of the brain of P. planeri 

 given by Wiedersheim 5 ) agree closely with the brain of L. mitsukurii 

 of Japan. 



I accept mainly on account of the brain configurations just stated, 

 the. views of recent writers in maintaining that the genus Petromyzon 

 shpuld be split into two : retaining the old name Petromyzon for P. 



I . 



1) Gage, The Lake and Brook Lampreys of New York: Wilder's Quarter-century 

 Book, 1893. 



2) Jordan and Evermann, Fishes of North :md Middle America, Pt. 1, 1899. 



3) P. marinus has not been hitherto met with in Japan. 



4) Ahlborn, Untersuch, ü. d. Gehirn der Petromyzonten ; Zeitschrift f. w. Zool., Bd. 

 XXXIX, 1883. 



5) Wiedersheim, Das Gehirn von Ammocoetes und Petromyzon planeri ; Jen. Zeit- 

 schrift, Bd. XIV, 1880. 



