208 FRESH-WATER NEREIDS FROM THE PACIFIC COAST AND HAWAII. 



"This lake is a remarkable feature of the topography. It lies in a depression 

 which is a structural valley and which is separated from the ocean by a very narrow 

 ridge. Though thus lying in a structural valley, the form of the shore contours of 

 the lake, the stream cliffs by which it is bounded, and its relation to the drainage of 

 the valley all demonstrate that the immediate basin of the lake is a drowned valley 

 of stream erosion. 



"The bottom of Lake Merced is ten feet below sea-level, and this fact demon- 

 strates a recent submergence. The drowned valley undoubtedly once had free access 

 to the ocean at tide level, but sand-dunes choked the channel and dammed back the 

 waters till they stood ten feet above tide. Since then the lake has been artificially 

 raised another ten feet." 



I am indebted to Professor Lawson for calling my attention to his published 

 account of Lake Merced, from which the foregoing is taken; also for the statement, 

 in private communication, that "the date of the invasion of the salt water was in 

 late Quaternary time." This was probably the time when the marine life became 

 established in the lake. 



At present there is no access of the sea, and the water, for economic reasons, is 

 kept fresh at all times. The marine forms found in the lake may therefore be 

 regarded as its permanent and thoroughly established inhabitants. The vegeta- 

 tion and most of the animal lif e is such as occurs generally in the fresh waters of the 

 Northern Hemisphere; but along with the fresh-water creatures and apparently as 

 much at home as they, we find besides the Nereis two species of marine isopods and 

 one schizopod, Neomysis mercedis Holmes. The isopods, Sphseroma oregonensis 

 and Corophium spinicorne, as I have been informed by Dr. Holmes, not infrequently 

 enter streams and go up into fresh water, sometimes miles inland. 



II. NEREIS LIMNICOLA sp. nov. 



The living worm is a rich brownish-red in color, the red being due to the highly 

 developed vascular system and the brown to epidermal pigment which is especially 

 abundant on the antero-dorsal surface and its appendages. It is unpigmented 

 beneath. The size is moderate for a nereid. A large specimen measures 47 milli- 

 metres in length, 5 millimetres in transverse diameter, including parapodia, and 3 

 millimetres not including the parapodia. In small and medium specimens the form 

 is club-shaped, being thickest anteriorly, between the fifth and tenth somites, thence 



