FOSSILS OF COMO AND SHIRLEY STAGES 385 



Invertebrates 



Only a few species of freshwater invertebrates are known. These belong to the 

 genera Unio, Planorbis, and allied forms. Many of these fossils are new to sci- 

 ence and will shortly be described by Doctor E. H. Barbour. 



Plants 



CYCADS * 



A new genus represented by 20 new species, none of which have yet been pub- 

 lished. There are also numerous species of petrified wood. 



EXTENT OF COMO STAGE 



The Como stage is quite extensive, its northern limit being in Montana 

 and its southern at least in southern Colorado. These beds are known 

 in the Black hills and as far west as the 109th meridian. 



SHIRLEY STAGE AND ITS FOSSILS 



The marine division may hereafter be known as the Shirley f stage, 

 which is composed of bands of shale, limestone, sandstone, and clay. 

 The limestones are usually shaly. The limestone beds are quite thin, 

 but usually fossiliferous. The clays and shales usually contain large 

 concretions which contain both vertebrate and invertebrate fossils. 

 Septaria are common. The invertebrate as well as vertebrate faunas 

 are only partly known. This has been largely due to the fact that the 

 richest fossil localities are where the concretions are well developed, and 

 until recently not many of these were known. 



Invertebrates 



Some of the important invertebrate species $ that mark this stage are 



Lingula brevirostris, M. & H. Tancredia cf. extensa, White. 



Oslrea engebnanni, Meek. Tancredia warrenana, M. & H. 



Lima sp. Pholodomya kingl, Meek. . 



Lima n. sp. Thracia iveedi, Stanton. 



Complonecies extenuata, M. & H. Pleuromya sp. 



*It is only recently that eyeads have been found in unquestionable Jurassic rocks in America, 

 and this first discovery was made in the Freezeout hills during the summer of 1898 by a party of 

 the University of Wyoming. The collection made was not large, but has been placed in Doctor 

 Ward's hands, of the United States Geological Survey, for study, and he has already reported that 

 he has found in the collection one new genus and twenty new species of cycads. 



f Name taken from the Shirley mountains, on the south side of which this formation is very well 

 developed. 



1 1 am indebted to Doctor T. W. Stanton, of the United States Geological Survey, for the iden- 

 tification of the greater portion of this list. 



LV— Bum,. Geot,. Soc. Am., Vot-. 11, 1899 



