240 



GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



frequently occur in abundance on the surfaces of the deserts at a distance 

 from existing streams and lakes Some of these unquestionably lived in the 

 former lake during its last recession and .were left strewn over its bottom 

 when desiccation took place ; in many cases, however, the surface shells 

 are true fossils that have been separated from their matrix of clay and marl 

 and accumulated in certain areas by the action of the M'ind. Other "dead 

 shells," of which Pyrjjula Nevadensis is an example, have only been obtained 

 about the shores of the existing lakes and are probably still living in their 

 waters ; these are also termed semi-fossil. 



The thousands of shells obtained are all of fresh-water forms, and 

 include 27 species, grouped under 20 genera and 7 families. The genera 

 and species, together with the horizon from which they were obtained, are 

 indicated in the following table : 



Tahh of shells found in the Lahontan basin. 



Margaritana margaritifera, Linn. 



AnodoDta Niittallinna, Lea 



Splia-riuiii deutatuin, Hald 



sri iatiuuni, Lam 



I'isitlium ultianutntauuni, Prni 



fDnipiessum, Prm .- .. 

 Helisonia 1 rivolvis, Say 



umuiou, Gould 



Gyraulua parvus, Say 



vermicularis, Gould 



Menetus oporcubiiis, Gould 



LiniDSea stagnalia, Linn 



Linmophysa palustris, Mull 



bnlimoides, Lea 



i-)d 





►^g 



+ 



-I- 

 + 

 + 



+ 

 + 

 + 



.•a 



DD 



a 

 a 



+ 

 -I- 

 + 

 + 

 + 



+ 



+ 

 -1- 

 + 

 + 

 + 

 + 

 + 



Limnophyaa samassi, Bd 



humilis, Say . .. 



Physa bunierosa, Gould 



Pompliolyx cffiisa, Lea 



Carinifes Newberryi, Lea 



Aiicylus NewbLTryi, Lea 



Aranicola lonsinqua, Gould . 

 Pyrgula Nevadensis, Stearua. 



Fluminicola i'usca, Hald 



Valvata virens, Tryon 



Vallonia ]»ulcbella, Mull 



Succiuea strt-tcbiana, Bd 



Pupilla muacorum, Linu ... . 



►J a 1^ = 



s§ ill I el 

 5-^ I §•" , p-= 



iJd 



In arranging the fossils according to their geological horizons we have 

 considered the lower lacustral beds as, at least in part, contemporaneous 

 Avith tlie lithoid tufa ; the medial gravels have been correlated in time with 

 the thinolitic tufa ; and the upper lacustral clays with the dendritic tufa. 



As indicated in the list, only a single species {Pompholyx effasd) has 

 thus far been derived from the lower lacustral clays. This was found in 

 great abundance in the lithoid tufa on Anaho Island, and was equally com- 



