STUDY OP THE GRAPTOLITES. ' 189 



How far the bulb-like appearance at the base of some of the species of 

 Dendrogaptus may correspond to the disc or sac of G. bicornis and others, 

 I have not at this time the means of satisfactory determination. 



IV. NATURE AND PARTS OP THE STIPE PROPER. 



1. The solid axis. 



2. The common canal — comon body, or cwnosarc. 



3. The calycles or cellules [hydrotheca). 



4. Nature and ornaments of the test. 



1. The Solid Axis. — K\\ the graptolites proper have been found to be 

 provided with a slender solid axis,* while this feature has not been satis- 

 factorily proved in regard to Dictyonema, and to 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 celluliferous margin; and 

 in the branching forms it is subdivided, following all the ramifications 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 monoprionidian 

 species, of America, we have never found the axis prolonged beyond, or 

 denuded of, the cellules; as shown in G. colonus^ by Barrande, in his 

 Graptolites of Bohemia (Plate ii, fig. 5 of that memoir). In all the speci- 

 mens 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 fre- 

 quently seen extending beyond the celluliferous portion of the stipe at its 

 outer extremity, while the radicle is the continuation of the same below the 

 base of the cellules. The axis thus appears to be the foundation 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, however, which 

 present so great an extension of the solid axis beyond the common body, 

 the cellules may perhaps have been removed by subsequent 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 

 our 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. BARnANDE has ascertained that 

 this axis is solid and cylindrical, its diameter not exceeding \ millimotrc, and itd atruc- 

 ture apparently ^hioxxs^Uraptoiitcs de Bohcvic, page 4). 



