STUDY OF THE GRAPT0LITE8. 225 



undulation of the axis, corresponding to this enlargement of the parts in 

 the common body (Plate ii, figs. 10 and 11). 



In the diprionidian species, the cellules on the two sides of the stipe 

 are alternating, so that the bases or the apertures are opposite the space 

 between two others. This is more especially shown in figures 10 and 

 12 of Plate ii. 



In much the larger proportion of species, the body of the Graptolite 

 and the cellules are so extremely compressed, that they appear only as 

 serratures along the margin, with distinct impressed lines marking the 

 cell-divisions. The exterior margin of these serratures indicates in an 

 approximate degree the outline of the aperture ; and the frequently 

 occurring mucronate extension at the extremity of the cellule is produced 

 by the continuation of the cell-partitions, or sometimes by an outgrowth 

 from the margin of the stipe above or below the aperture. 



Were the cellules isolated, their prevailing form would be that of an 

 elliptical tube or sac, the length of which is greater than either of the 

 two diameters. When they are in juxtaposition, however, the contiguous 

 sides are flattened, while the lateral or external surfaces are usually 

 more or less curved, particularly near the aperture. In a larger propor- 

 tion of the species, the calycle becomes slightly expanded towards the 

 aperture ; but in a few examples there is a distinct contraction above 

 the middle, and the aperture is smaller than the base. Generally, 

 however, the smaller diameter is just at the junction with the com- 

 mon body, or at the junction of the cell- walls with the walls of the 

 common canal. 



In a single diprionidian species, where the specimens are not distorted 

 by pressure, a longitudinal section of the stipe in the direction of its 

 greatest diameter (Plate ii, fig. 12), shows the cellules scarcely narrowed 

 at their origin with the common body ; while in a lateral view of the 

 specimen, the base of the cellule is seen to be much wider than the orifice 

 (Plate ii, fig. 10). 



In many of the species a transverse section of the cellule near the 

 base is quadrangular, becoming more rounded towards the aperture ; and 

 when the upper part of the cellule is free, the aperture is round or ellip- 

 tical, and in some specimens the calycle is elliptical or cylindrical 

 throughout its entire length. We have examples of the quadrangular 

 cellules in G. extensus (Plate iii, fig. 12) and G. odobrachiatus (Plate iii, 

 figs. 25 and 27) ; as well as in one species of Phyllograptus (Plate iv, figs. 

 1, 2, 4 and 5). Where the cellules are more nearly isolated, they 

 Cab. Nat. 29 



