Dr. 0. Herrmann — Distribution of Grajitolites. 407 



side new genera make their appearance, culminate and disappear. 

 Even after the other Graptolites had long since disappeared from the 

 ancient sea-fauna, this genus still lived on, for we find it occurring 

 in the Devonian. 



All other Graptolites, however, are confined to the three subdivi- 

 sions of the Silurian : — the Cambrian, Lower Silurian (Ordovician), 

 and Upper Silurian (Silurian System ; Silurian proper). 



If we wish to obtain a clear and complete picture of the vertical 

 distribution of the Graptolites, we find the course made plain by G. 

 Lapwoi'th's classical memoir, " On the Geological Distribution of 

 the Bhabdophora.'' ^ From this invaluable work it appears that not 

 only the famihes and genera, but even the individual species are 

 confined to perfectly definite levels ; that the zones characterized by 

 particular genera or species occupy the same relative position to one 

 another over the whole earth ; and that the Graptolites are well fitted 

 to enable us to make even a detailed subdivision of the Silurian for- 

 mation. 



Graptolites are known from the Upper Cambrian. G. Linnarsson 

 has described a branched species of Graptolite^ from the Olenus- 

 shales of West Gothland ; Bryograptus Callavei, Lapw., has been 

 found in England in somewhat younger deposits ; and in Norway 

 Cambrian strata have furnished Dictyograptus flabeUiformis, Eichw., 

 and Bryograptus Kjervlfi, Lapw. (and other species of these two 

 genera), in great individual abundance. The question now arises : — 

 Which of the species of Graptolites enumerated is the most ancient? 

 Lapworth answers this question to the efi"ect that D. tenelliis, Linnars., 

 and B. Callavei, Lapw., with B. Kjerulfi, Lapw., open the series of 

 the Graptolites. On the contrary, Brogger^ shows that in Norway, 

 B. Kjerulfi, Lapw., occurs in the upper beds of the Dirtyograptus- 

 shales, over Diet, flahelliformis, Eichw., itself, and proves that this is 

 also the case in England with S. Callavei, Lapw. The zone with 

 Bryograptus, Lapw., is therefore in his opinion younger, both in 

 Norway and in England, than the zone with Dictyograptus, Hopk. 

 But we must regard the Swedish Dichograptus tenellus, Linnars., as 

 older than Dictyograptus flahelliformis, Eichw., and indeed as the 

 oldest Graptolite which has as yet become known to us. According 

 to Linnarsson, it is contained in the OZe?^MS-shales (Peltur a -zone) of 

 West Gothland, which is overlain by the Dictyograptus-shales ; 

 according to Von Schmalensee, it occurs above the Peltura-zone, but 

 in deposits which are in contact therewith. Dictyograptus tenellus, 

 Linnars., belongs undoubtedly to the family Dichograptidae, Lapw., 

 and this family is in fact the oldest family of Graptolites. 



Any one who is for the first time occupied seriously with the 

 sequence of the Graptolites in age will be exceedingly astonished at 

 such a result, as he does not find those forms which are the most 



1 Ann. Mag. Nat. Hist. ser. 5, vol. iii. pp. 245 and 449 ; vol. iv. pp. 333 and 

 423 ; vol. V. pp. 45, 273, and 358 ; and vol. vi. pp. 16 and 185. 



^ It received from Linnarsson the name of Iticliograplus tenellus, L., but errone- 

 ously. Upon this further hereafter. 



^ Loe. cit. pp. 145. 



