420 Forty-second Report on the State Museum. 



Kayseb, Novak, and Barbois; so that the essential difference in the 

 views of Barrande and his critics lies, not so much in the determina- 

 tion of the faunistic equivalents of F, G, H, as in the secular value of 

 their faunal characters; a case of Devonian vs. post-classical Silurian. 



In New York, as in Bohemia and the Carinthian Alps, the passage 

 from the Silurian to the Devonian is interrupted by no great defaults, 

 and it really becomes a question of very little moment where, in any 

 country, such a line is to be drawn. With increasing knowledge of 

 the fossil contents of the earth, we may look forward to a cementing 

 of the breaks in the record of organic succession which are now con- 

 ventionally represented by lines, or planes, or stories in the house of 

 life, as with the rapidly increasing additions which every year brings 

 to our knowledge of fossil organisms, it becomes more and more difficult 

 to adhere to any conventional system of classification. The essential 

 point is to determine when and under what circumstances of sedi- 

 mentation and association organic types of generic or broader value 

 have appeared, what has been the mode of development, variation and 

 extinction of such types, and what the history of the fauna; how 

 modified by migration, by changes in sedimentation or depth of sea 

 bottom. 



In turning our attention briefly to the consideration of what Ameri- 

 can fauna comprehends the strongest alliances with the Hercynian 

 or lowest Devonian pelagic fauna, it is necessary to notice the impor- 

 tant observations on this point by all the principals in the Hercynian 

 discussion. [Reference to these has been intentionally left to this place. 



We have just noticed that Barrande considered the Upper Helder- 

 berg as a Silurian fauna, the nearest equivalent of his faunas F, G, H, 

 especially of G. Beyrich (1867), in suggesting the Devonian age of 

 the lower Wieda limestones and the possible equivalence to the Bohe- 

 mian F, G, H, extended the parallel to the calcareous facies of the 

 " Helderberg Division " of New York. Kayser (1878), in his descrip- 

 tion of Hercynian species, made many identifications and comparisons 

 which it is essential here to notice in detail. 



Trilobites. To the group of Dalmanites Hausmanni are referred: 



D. pleuroptyx L. H. D. myrmecophorus U. H. 



D. micrurus L. H. D. Helena U. H. 



D. tridens L. H. D. Calypso U. H. 



D. nasutus L. H. ? D. Erina U. H. 

 D. tridentiferusL.H. ? D. acanthopleurus (= myrmecophorus) U. H. 



? D. denticulatus S. G. 



? D. emarginatus S. G. 

 D. Beyrichi, Kayser, to the group of D. pleuroptyx. 



