368 Graptolites. 



having a structure so very different, he proposed the name 

 Glimacograptus. The Linnaean Graptolithus scalaris belongs to 

 this restricted genus, and Hall, who was aware of this, recognized 

 the original specific designation in proposing the new name. 

 He is not, however, aware that he is here dealing with the 

 only species which Linnasus knew, but, like all other writers, 

 he erroneously considers Hisinger's Priodon Sagittarius to be 

 the Linnasan type of the family. 



Some species of Didijmograpsus have this polypary 

 extended into a double-celled prolongation below the point 

 of union of the two diverging branches (Plate II., Fig. 13), 

 and these Hall names Dicranograptus , chiefly, however, because 

 they have, as he thinks, the same remarkable structure as 

 Climacograptus . 



Having thus traced historically the establishment of the 

 different genera of graptolites that have been found in Britain, 

 we shall better understand their relation if we arrange them 

 in systematic order. 



Section I. — Species with a single series of cells. 



1. Rastrites, Barrande. Polypary simple, with slender 

 tabular cells, rising more or less at a right angle from the 

 delicate capillary axis, and having their bases separated by a 

 considerable interval. There are five species of this genus 

 found in Britain, all of them from the Llandeilo rocks. The 

 fossil named by Harkness, R. triangulatus, is the older portion 

 of Graptolithus convolutus, His. (Fig. 15). Prof. Wyville 

 Thompson has pointed out to me that the earliest portion of 

 this species was composed of distant tubular cells, exactly 

 resembling those of Rastrites. I find in my own collections 

 specimens which confirm this opinion. The cell mouths were 

 furnished with two spine-like appendages (Plate I., Fig. 17). 

 It cannot be determined whether these larger cells had any 

 special functional office to discharge. Two species are figured 

 on Plate II., B. Linnmi, Barr., Fig. 9, and R. capillaris, Car., 

 Fig. 10. 



2. Graptolithus, Linn. Polypary simple, with the cells 

 rising at an acute angle, and in contact throughout more or 

 less of their length. Fourteen British species are known, the 

 majority from the Llandeilo rocks, four from the Caradoc, and 

 one, G. priodon, Bronn, rising up into the newest beds in 

 which any true graptolite has yet been found — the Ludlow 

 series. Fig. 1 is G. Sedgwickii, Portl., originally described 

 from the north of Ireland, but found also in Scotland and 

 Wales. Fig. 3 is G. Hisingeri, XJar., which is Hisinger's 

 Priodon Sagittarius. To prevent further confusion, as well as 

 to correct an error, I have set aside the false Linmean specific 



