26 CANADIAN FOSSILS. 
| 
RAstTRITES BARRANDI: 
Fig. 28. 
natural size, with a portion enlarged. The cellules consist of long slender tubes. 
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 the enlarged figures 
10 and 12, plate A. 
In much the larger proportion of species, the body of the graptolite and 
the cellules are so extremely compressed, that they appear only as serra- 
tures along the margin, with distinct impressed lines marking the cell- 
divisions. The exterior margin of these serratures indicates in an approx- 
imate 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-partition, 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 proportion 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 common body, or at the junction 
of the cell-walls with the walls of the common canal. 
In asingle diprionidian species, where the specimens are not distorted by 
pressure, a longitudinal section of the stipe in the direction of its greatest 
diameter (plate A, fig. 12), shows the cellules scarcely narrowed at their 
origin with the common body; while in a lateral view of the specimen, 
