INTRODUCTION. xvii 



The continuance of this fauna is almost entirely interrupted by the coming 

 in of the Genesee slate and the Portage shales and sandstones in central and 

 western New York, and is abruptly terminated in the eastern part of the State 

 by the sedimentary accumulation of the Oneonta sandstone. 



The Portage group, in some of its typical localities, carries a few forms of 

 Lamellibranchiate shells, some of which, so far as we now know, have no 

 generic relations with the prevailing forms above or below this horizon. 



The genus Panenka and Lunulicardium, as well as the cosmopolitan form 

 Glyptocardia, also occur in the Portage group. Thus far the pectenoid or 

 aviculoid forms are not known in the rocks of this age, but our knowledge is not 

 yet sufficient to speak positively of its entire fauna. Moreover, the western 

 extension of this group has not been satisfactorily determined, and we are 

 unable to present any facts regarding the distribution of species of that age 

 beyond the State of New York. 



Following the final deposition of the two latter formations, the Lamelli- 

 branchiate fauna reassumes its prominence under a somewhat modified aspect. A 

 few of the forms which were predominant in the Hamilton group have become 

 subdued in force, while others are augmented. Among the Monomyaria, there 

 is a notable increase in the number of pectenoid and aviculoid forms, espe- 

 cially of Pterinea and Actinopteria. 



In the formations below the Chemung, the genus Leptodesma is represented by 

 two species, while the total number described is fifty-seven. Leiopteria, on the 

 other hand, shows its greatest development in the Hamilton group. Pteronites 

 proper is unknown below the Chemung, and the genus Ptychopteria, with its 

 twenty-two species, is also unknown below that horizon. 



Among the pectenoid forms there is a larger number of species of Pterino- 

 pecten below the Chemung, while Crenipecten, with its eight species in the 

 Chemung group, is also unknown to us in any lower formation. 



The genus Glyptodesma, so abundant and varied in form in a single species 

 in the Hamilton group, is at present unknown in any higher formation. 



Pterinea flabella, beginning its existence at the horizon of the Corniferous 



