628 GEOLOGY OF OHIO. 



obtained in nature. Opinions will always vary as to what degree of dif- 

 ference is required to establish a genus, but all agree that it is a larger or 

 smaller group of closely related species. The first essential then is to 

 show that the supposed generic characters exist in two or more specifi- 

 cally distinct forms — the more the better for the genus. 



Now, it happens that a majority of the new genera founded by me 

 upon shells of the Cincinnati group have earlier representatives in the 

 Trenton limestones of the Northwest, and these as well as the genera 

 are described in the Minnesota work referred to. The real types and 

 bulk of the genera, however, could find no place in that work, and I do 

 not doubt that their publication would have been postponed for years had 

 the present opportunity not become available. That it did is most fortu- 

 nate, since I was thereby enabled to give a degree of thoroughness to 

 my studies that would not have been possible had I been obliged to 

 depend entirely upon my own resources. This thoroughness lies chiefly 

 in the delineation and comparison of the generic groups in accordance 

 with facts gathered in a study of collections containing a large number 

 of hitherto unknown forms. By itself the Minnesota work is probably 

 insufficient in its specific part to establish all the innovations proposed, 

 and it is therefore again fortunate that the date of the present publica- 

 tion will be little if at all subsequent to that of the Minnesota volume. 

 Between the two it is hoped that the desired completeness may be 

 obtained, and that the validity of some of the new genera will be estab- 

 lished almost by force of numbers alone. 



So far I have published nothing on the Avicalida>, although the 

 Lower Silurian forms have been studied thoroughly and drawings of 

 many of them prepared. Some important facts relating to their preser- 

 vation have been discovered, and sufficient new material to make an 

 interesting paper by itself, is already available. Aside from the avicu- 

 loids there remains also a considerable number of Lamellibranchiata that 

 have never been described. Indeed, of nearly every L,ower Silurian 

 genus established previous to 1890 from one to as high as six and seven 

 undescribed species are known, and with every j*ear the number is in- 

 creased. I have not therefore by any means exhausted the subject even 

 in its specific part, and when it comes to the classification of the species 

 and genera we are really but little beyond a good beginning. And, ac- 

 cording to my views, this is just as it should be. I go namely from the 

 standpoint that the higher classification must evolve itself, and come, if 

 the expression be allowed, as a by-product from our studies of individual 

 and specific forms, and finally of the generie types. 



The plates accompanying the present work were prepared b}* myself. 

 The specimens used, except where the}* are credited to other parties, are 

 to be understood as belonging to my private collection. 



