xxvi BRITISH GRAPTOLITES. 



Ricliter, tlio occurrence of sucli numbers of specimens on tliese extremely thin 

 layers of rock indicates that the Graptolites were very short-lived, and that the 

 fact that they gradually disappear as the Alum shales thin out, shows that they 

 were confined to deep water. 



A remarkable paper published by Scharenberg in 1851, 



c, , ,' which may be regarded in a sense as a direct outcome of 



' Ueber Graptolithen, Barrande's memoir, is of great importance, the author 



mit besonderer Beriick- criticising fully the observations and views of Barrande in 



sichtigung der bei the light of his own researches among the Graptolites from 



Christiania vorkom- ^j^^ Christiania district of Norway. 



menden Arteu.' t. ■ , ■ , , . ^ . . ^ , , 



Vescvvption oj tSpeaes. — In this paper bcharenberg 



describes and figures six species : (1) Gr. geminu>f, (2) Gr. vinjnlatiis, (3) Gr. 

 folium, (4) Gr. teretlusculus, (5) Gr. Barrandei, and (6) Gr. personatus, of which 

 the last two only are new. The figures are conscientiously drawn, every 

 imperfection being represented, and are consequently most valuable. But the 

 extreme caution by which not only the figures, but also all the details in his 

 work are characterised, leads him to include under one specific name several 

 forms now regarded as specifically distinct (although, on the other hand, his 

 own two supposed new species cannot be sustained). Under the name of Gr. 

 teretiuscidus he included at least two distinct species. One of these is the character- 

 istic Arenig-Llandeilo species of Glhnacocjraptus (subsequently named after him by 

 Lapworth as G. Schareubergi). The specimens described by him as Gr. folium 

 are examples of riiyllograptus. 



Sjoecific Griteria. — Scharenberg prefaced the descriptive portion of his work 

 by a careful analysis of the most important points to be considered in distinguishing 

 different species. As these are of considerable interest, they may here be briefly 

 considered. 



1 . Length of Stijje. — According to Scharenberg this character is not of much 

 importance, owing to the fact that it is difficult to determine when the specimen 

 is complete, and that even fully grown specimens of one and the same species may 

 vary much in size. 



2. Width of 8ti2Je. — On the other hand, Scharenberg considers that there is 

 a practically constant relation between the width of the cells and the common 

 canal. 



8. General Form of the rohjpdnj. — Scharenberg regards this character as one 

 necessitating extreme caution in its interpretation, concluding his remarks on the 

 matter as follows : — " The determination of a species founded on the outer form of 

 a Graptolite is only trustworthy when there is only one row of cells, and tliese 

 cells are distant (Bastrites) . Teeth, denticles, and appendages of the cell must be 

 considered as distorted or falsely shaped cell apertures, until their normal condition 

 has been proved with certainty in Avell-preserved specimens, in the same way as 



