PLIOCENE FRESH WATER FOSSILS. 169 



small form being the only one yet found fossil in any lo- 

 cality. It is even smaller than that found living in Clear 

 Lake, which I have pubhshed as var. minor. No. 3 a sin- 

 gle broken specimen of the heterostropha group, very un- 

 like P. costata. No. 4 is extremely abundant, often form- 

 ing masses by itself. It is not now known to inhabit any 

 of the neighboring streams, but is fluviatile in habits, and 

 as most of the mountain streams dry up entirely in some 

 years, it may have been thus exterminated south of Mon- 

 terey Bay. 



The fresh water beds of this valley are still scarcely 

 touched, and probably extend over a very much greater 

 surface. They are not usually well exposed, and diffi- 

 cult to explore. They might be called miocene from 

 their high dip, but this was no doubt from local causes — 

 landslides, etc. 



The Contra Costa Lake Bed. 



This hes chiefly on the northeast slope of the hills west 

 of San Pablo Creek, forming the boundary between 

 Contra Costa and Alameda counties in that part of its 

 course, about 4^^ miles northeast of the State University. 

 A thin stratum of lignite was found there exposed on the 

 side of a small branch of the main creek along the road 

 going east to Lafayette, and tunnelhng into the side of 

 the hill exposed it in places farther north. It has a dip 

 toward the northeast of about 30°, and probably contin- 

 ues eastward under the valley, though not yet seen there, 

 as the value of the lignite would not pay for boring. 



In the shaly layers of Hgnite I found a few fossils, of 

 which figures are here given, and which I call: i, x\no- 

 donta nuttaliana Lea.? var. hgnitica J. G. C, plate xiv, 

 fig. II. 2, Linna^a contracosta J. G. C, plate xiv, fig. 

 12. 3, Planorbis pabloanus J. G. C, plate xiv, fig. 9, all 

 of natural size. 



