60 Art. 2.— T. Kaburaki: 



Tajima) or in high mountain regions. In general, it abounds at a 

 higher altitude than Poly, auricidata. Bd. hrvnnea is met with in 

 cool running brooks chiefly in the northern mountain districts. 

 The foregoing species are not found in the warmer regions such as 

 Kiushu and Shikoku. PI. yenocephala, wliicli is of wide distri- 

 bution in the northern hemisphere, has the greatest range, extend- 

 ing from the mountainous region dowii to the sea level and usually 

 inhabiting streams or lakes of a liigher temperture than the streams, 

 in which PL viv'ida and Poly, auriculata occur. At the high levels 

 PI. yonocefhala occurs sporadically, and at the highest levels not 

 at all. 



Now, let us consider the conditions affecting the distribution 

 in the Nikko region wliich includes the town of Nikko, Chuzenji 

 and Yumoto, extending from an altitude of 500 m. to one of 1G20 

 m. above the sea level. The temperature diminishes steadily as we 

 ascend towards Yumoto, where people do not reside in winter. 

 Extensively distriluited in this region are found three species, PL 

 gonocejdiala, PI. vivida and Poly, auriculata, the distribution of 

 which shows some interesting features. My investigations show 

 that four distinctly marked sub-]-egions can be distinguished. The 

 first sul)-region is represented by the vicinity of the town of Nikko, 

 which affords positions favourable to PL gonocqihala. This species 

 is very alnmdant in the streams flowing through this locality. The 

 second sub-region extends from tlie neighbourhood of Nikko to 

 an altitude of about 833 m., and in it the three species just men- 

 tioned are found together. Ascending the mountains we enter 

 the third sub-region in Chuzenji, standing at an altitude of about 

 1350 m., and liere flourishes Poly, avricvlata. We come next to 

 the last sub-region in the neighbourhood of Yumoto, whei'e PL 

 vivida occurs fairly abundantly in cool running 1 »rooks. Here 

 also is to be found PL gonoce'phala, but it is confined either to the 

 riveis or to lakes. 



Associated with each other the three species, PI yonocej)hala, 

 PI vivida and Poly, auriculata are to be found in the second sul)- 

 region mentioned above. Their distribution suggests that there is 

 between them a struggle for existence. But a close ol)servation 



