470 NJ^' YORK STATE MUSEUM 



Sedgwick, who [1848J described graptolites from the Skiddaw slates in north 

 Euo'land ; Salter, who [1848] recognized some of Hail's Normanskill shale 

 forms in the slates of Loch Ryan, described other species from the rocks of 

 Scotland in 1851 and 1852, in 1861 proposed the new genus Dichograptus, 

 and in 18(53 Tetragraptus ; McCoy, who erected the genus Diplograpsus 

 [1850] and described numerous species from British paleozoic rocks, recog- 

 nizing several American species among them, in the British Palaeozoic 

 Fossils [1851]; and Harkness [1850], who described the graptolites of the 

 black shales of Dumfriesshire, a work continued by CarrtitTiers in 1858. 



On the continent xS'?(6'S« [1851] added considerably to the list of forms 

 made known by Barrande from Bohemia; and Geinitz [1852] described the 

 graptolites of Saxony. 



In America Hall's work found early response. Billiiujs [1861] compared 

 the zones of Europe and America and endeavored to show that the graptolite 

 shales of Normanskill near Albany were not in the upper part of the Lower 

 Siluric, or Hudson river group, as Hall maintained, a contention which 

 is now decided in Billings's favor [[).490]; and Logan [1863] recognized 

 the occurrence of graptolites of the Normanskill and Utica shale of New York 

 in Canadian rocks. 



The remarkable zonal distribution of the British graptolites was however 

 not fully recognized nor understood, till Nicholson, Hopkinson and, specially, 

 Lapworth began their systematic exploitation of the British graptolite shales. 

 By their endeavors it became known that six principal divisions can be dis- 

 cerned in the graptolite shales of the Upper Cambric and Siluric, that these 

 again can be subdivided into smaller aubzones, and that these zones show an 

 astonishing uniformity of succession throughout the British Isles. 



When finally the same work was undertaken in Scandinavia by Linnarsson, 

 it was found in Scania, where the graptolite shales are greatly developed and 

 little disturbed, that the same principal divisions as in Great Britain could be 

 recognized ; and Tidlberg and Tornquist were able to subdivide the Swedish 

 Siluric by means of the graptolites so minutely that in all stratigraphy we find a 



