STUDY OF THE GRAPTOLITES. 219 



1. THE SOLID AXIS. 



All the Graptolites proper are provided with a slender solid axis,* 

 while this feature has not been satisfactorily proved in regard to Dictyo- 

 nema^ and some other forms. 



In those species having a single series of cellules, this axis is upon the 

 back of the stipe, or on the side opposite to the celluUferous margin ; 

 and in the branching forms it is subdivided, following all the ramifica- 

 tions and holding the same relation to the cellules. In all the specimens 

 where it has been observed, it is a slender cylindrical or flattened filiform 

 solid body. In some extremely compressed specimens this axis appears 

 as a slender elevated ridge along the back of the stipe ; and where the 

 substance of the body has been removed, it leaves a narrow groove along 

 the margin of the impression. 



In the examination of large numbers of specimens of the monoprio- 

 nidian species, of America, we have never found the axis prolonged 

 beyond, or denuded of, the cellules ; as shown in G. colomis, by Barrande, 

 in his Graptolites of Bohemia (Plate ii, fig. 5 of that memoir). In all 

 the specimens where the extremities of the stipes are entire, there is 

 never any extension of the axis beyond the last partially developed 

 cellule ; and the number of specimens in this condition is considerable. 



In the Graptolites with two series of cellules, the solid axis is very 

 frequently seen extending beyond the celluUferous portion of the stipe 

 at its distal extremity, while the radicle is the continuation of the same 

 below the base of the cellules. The axis thus appears to be the founda- 

 tion on which the other parts are erected, being in these forms a condition 

 of their growth, and existing from the first appearance of the germs as 

 shown in several figures upon Plate i. In those older specimens, how- 

 ever, which present so great an extension of the solid axis beyond the 

 common body, the cellules may perhaps have been removed by subse- 

 quent causes ; or, in such cases, the absence of this extension of the 

 axis may be due to its having been broken off. It should be remarked, 

 however, that none of om- specimens having this character show any 

 evidence of the breaking or tearing away of the cells, nor do the cells 

 on one side extend beyond those on the other ; a condition we should 

 expect to find, had they been partially removed by maceration. 



* In those species with a single series of cellules, M. Barrande has ascertained that this axis 

 is solid and cylindrical, its diameter not exceeding | millimetre, and its structure apparently 

 fibrous (^Graptolites de Boheme, page 4). 



