23,2 H err e: Philippine Eels 137 



42 ; interorbital space, 23. I have also examined a second speci- 

 men from Lake Lanao, having a length of 795 millimeters. 



This species is common in Lake Lanao and has been recorded 

 by Casto de Elera from Manila. Lake Lanao is a large fresh- 

 water lake in northern Mindanao, having an area of 176 square 

 kilometers, and lies at an elevation of slightly more than 670 me- 

 ters. It is very deep along the southern shore, the military 

 finding a depth of over 150 fathoms near Bayong when laying the 

 cable which crosses the lake. The only outlet is through the 

 Agus, a small river at the northern end of the lake, only about 30 

 kilometers in length, and over a large part of its course a contin- 

 uous rapid. Near the coast it takes a leap of 58 meters, the 

 Maria Cristina Falls. As fresh-water eels spawn only in the 

 ocean, this presents a very interesting problem. It is evidently 

 impossible for any fish either to ascend or to descend these falls 

 directly, but a recent visit to Lanao offers a reasonable explana- 

 tion of the puzzle. 



Eels occur in such isolated lakes as NunuiTgan, which is entirely 

 surrounded by mountains but which has a subterranean outlet. 

 Through this channel eels and small Cyprinidae come and go. 

 The whole volcanic plateau of Lanao has many such subterranean 

 water courses. 



An inspection of Maria Cristina Falls shows that the lower 

 gorge of the Agus terminates in a box canon, the falls being 

 caused by the river leaping from the upper valley over the preci- 

 pice. On the south side of the gorge, perhaps a third of the way 

 above the torrent at the bottom, is a layer of rock which is 

 evidently honeycombed with water passages. From this layer 

 issue many small cascades and spurting streams which tumble 

 down the talus blocks into the boiling river below. Owing to 

 the overhanging nature of the cliffs on the north side, where 

 only one can safely view the falls, it is impossible for me to say 

 whether there is a similar layer of cavernous water-bearing rock 

 on that side or not. There can be no doubt that very slender 

 young eels crawl up the wet talus, enter the crevices in the 

 water-carrying layer of rock, and work their way up against 

 the subterranean streamlets until they reach the river well 

 above the falls. . 



The relative proportions of length, depth, head, eyes, interor- 

 bital space, snout, and mouth vary considerably with age and 

 stoutness, large specimens being relatively more robust, with 

 smaller eyes and larger mouth. According to Weber and Beau- 



