4 PREFACE. 



references to two places of publication, especially where, in the first instance, 

 the species was defined without illustration, as has been too frequently done 

 in society publications, the place above and beyond all others where no 

 species should be described unless accompanied by proper illustrations. 



An attempt has also been made to correct the misspelling of words so 

 as to perfect the nomenclature, and we call special attention to the Index of 

 Genera, where a few corrections are made that were overlooked in the text, 

 and where the gender of each genus is indicated. 



After the author had commenced the preparation of this work, which 

 was several years ago, knowing the great expense attending the making 

 of illustrations, he applied to several State Geologists and others for the 

 privilege of taking electrotypes from the wood-cuts belonging to the State 

 Governments and to the individuals ; and he has now to express his 

 acknowledgments to Alfred R. C. Selwyn, F. R. S, F. G. S., Director of 

 the Geological Survey of Canada, who placed at his disposal all belonging 

 to the Canadian Survey, and he availed himself of about one hundred and 

 sixty of the original figures used by the late Prof. Billings ; and also to 

 express his obligations to the late Prof. A. H. Worthen, from whom he 

 obtained nearly all those used in the Geological Survey of Illinois. After 

 a very large number of figures had been made by the expensive process of 

 wood-engraving, he learned of the much cheaper electrotype process, and 

 engaged the services of the Kline Photo-engraving Company, of Cincinnati, 

 and for the accuracy and faithfulness with which many figures have been 

 reproduced he is indebted to the skill of the artists in that company. 



Cincinnati, November, 1889. 



