FOSSILS OP THE KNOXVILLE BEDS. 447 



KXOXVILLE FAUNA. 



The beds immediately below this zone contain essentially the same 

 fauna,'with the addition of large numbers of Aucella (the most character- 

 istic fossil of the Knoxville) and a few other forms. At a locality about 

 two miles north of Wilcox's ranch the following fossils were found in 

 the uppermost strata of the Knoxville beds : 



AiiceUa crawicoUis^ Keyiierling; abundant. Ammonites (Lytoccras) hateni, Trask. 



Aiicdla piocliu , Gabb, \2iY ovata ; rare. Ammonites [OlcostepJianus), sp. a. 



Inoceramus, sp, undetermined. Crioceras lotus, Gabb. 



Helicaulax hicar'niata, Gabb. Belemn'des impressus, Gabb. 



The Aucellse play so important a part in the Knoxville fauna that it 

 is necessary to insert here a few notes on their variations and strati- 

 graphic range. 



Aucellx of the Knoxcille Beds. — The most characteristic and by far the 

 most abundant fossils of the Knoxville beds are the various forms of the 

 genus Aucella. In many places the rocks are completely filled with them, 

 and they are known to range through almost the entire great thickness 

 of the beds. These AuccILt show a great variety of forms and sizes, but 

 they are easily separable into two groups that are in a general way 

 characteristic of different horizons. One of these groups, which includes 

 the typical form of Aucella piochil, Gabb,^ is characterized by its slender, 

 oblique form, and is very inequivalve, the left valve being convex, with 

 a long, narrow, incurved l)eak, while the right valve is nearly flat, and 

 its beak also is narrow. Occasionally a si)ecimen shows faint, radiating 

 striae, suggesting the older Aucella erringtoni of the Mariposa beds. Of 

 the Russian species, Aucella mosquensis is its nearest relative, but as there 

 are several differences in form, especially of the umbonal region, it is 

 preferal)le to leave them under Gabb's name, which was proposed about 

 thirty years ago. 



At the lowest horizon in the Knoxville beds from which we have col- 

 lected in this region the typical Aiicella piochil occurs alone, but in higher 

 strata, about 3,000 or 4,000 feet below the top of the Knoxville, it is asso- 

 ciated with a larger, more convex variety that may be compared with 

 Aucella mosquensis, var. ovata, or with Aucella terehratuloides.f This variety 

 occurs sparingly at first, and at a horizon about 1,000 feet higher it be- 

 comes the pr(!vailing form. For convenience this variety is called Aucella 

 piochil, var. ovata, but it should be stated that there is great individual 

 variation, so that it is difficult to assign limits to any variety or species 

 that may be proposed. 



* Figured in Paleontology of California, vol. i, pi. 2'>, fig. 17-3. 



fSee Lahusen, J.: Ueber Ra.ssi.schen .\ucellen, Mem. Com. Geol., vol. 8, no. 1-, St. Peter.sburg, 

 1888, where the Russian species are all figured and described. 



