I. INTRODUCTION. 



At different times during the past seven years specimens of three undescribed 

 species of fresh-water Polychseta, all belonging to the Nereidso, have come into my 

 hands. Since unusual interest attaches to any form of animal life existing in an 

 environment so different from that of the majority of its congeners, I have thought it 

 worth while to give a brief account of these nereids. 



For the material I am primarily indebted to Dr. Gustav Eisen, of San Francisco. 

 From him I received in 1895 a specimen of a small Nereis which he had collected at 

 Lake Merced, near San Francisco. This led me to explore the lake, with the result 

 that this species, unlike any Nereis yet discovered in the neighboring sea, was found 

 in abundance. Subsequently Dr. Eisen placed at my disposal a minute nereid which 

 he had collected in 1892 in a mountain stream of the Sierra Laguna, Lower Cali- 

 fornia, at 7000 feet above sea-level. This species is of peculiar interest, not only on 

 account of its extraordinarily elevated habitat, but because it is the type of a new 

 genus almost perfectly intermediate between the very distinct and isolated genera 

 Lycastis and Ceratocephale. The first specimen of the third form, a new species of 

 Lycastis from the Hawaiian Islands, was also the gift of Dr. Eisen. Additional 

 specimens of this species have at my request been collected and sent me by Professor 

 L. H. Miller of Oahu College, Honolulu. It is a pleasure to express my grateful 

 acknowledgments to Dr. Eisen and Professor Miller for their great kindness in sup- 

 plying me with this material. 



There is no reason to doubt that all three species live in perfectly fresh (drinkable) 

 water. Lake Merced is a part of the water system by which San Francisco is sup- 

 plied; Lycastis hawaiiensis is reported by Professor Miller as living in a spring; while 

 the elevated habitat of the species from the Sierra Laguna is sufficient guarantee 

 that the water is not brackish. Unfortunately, further data concerning its interest- 

 ing habitat are lacking. 



Lake Merced, the only known habitat of Nereis limnicola, is a body of water of 

 irregular shape about two miles in length occupying a submerged river valley among 

 the sand-dunes of the San Francisco peninsula. It is within a quarter of a mile of 

 the ocean, from which, according to Lawson, it has in very recent geological time 

 been shut off by the shifting sands. Lawson ('95, p. 474) says in regard to it: 



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