GRAPTOLITES. 29 
and this appears to be true of all the species with a single range of cellules, 
and also of the ordinary forms of those with two ranges of cellules, where 
the common body. is divided by a longitudinal axis. 
In Rettolites, where there is no well-marked division limiting the com- 
mon body, the union of the cellules with it is not so well defined; nor 
does there appear to be in these forms a continuous cell-partition: the 
cellules open in a quadrangular aperture, which is a little oblique to the 
transverse diameter. 
In those graptolites with the simple transversely-oval orifices in the test, 
as G. bicornis, the arrangement of the common body and the communica- 
tion of the cellules differ from all the other forms. ‘There is an apparent 
double communication with the common body, giving not only the usual 
bilateral arrangement of the parts generally, but a bilateral arrangement 
of the parts in the individual alveoles. 
The external orifice of the cellule in graptolites is extremely variable in 
form, and in its relative direction to the body of the cellule, and to that of the 
general axis. In a large proportion of the species, the aperture is oblique 
to the axis of the cellule, a little expanded, and thickened at the margin. 
The lower or posterior edge is often prolonged into a mucronate point or 
expansion. ‘This feature, combined with the various degrees of curvature 
at or near the aperture, produces a great diversity of external expression 
in the orifice. 
In G. nitidus, plate i, and in G. similis and G'. extensus, plate ii, the 
plane of the orifice is nearly rectangular to the axis of the cellule; while 
in G. bifidus, G. pennatulus, and others, the margin is produced into a 
strong mucronate extension. In G. octobrachiatus the line of the cell- 
margin makes an angle of more than 90° with the axis of the cellule. 
In mature individuals of G. Clintonensis the upper part of the cellule is 
re-curved, and the orifice opens downward nearly at right angles to the 
general axis, having a slight spreading and thickening of the border. In 
less mature individuals the orifice is apparently angular, and opens upward, 
while the plane of the aperture makes less than a right angle with the 
direction of the general axis. It would appear that in the progress of 
growth the cell-walls are continued, gradually contracting above, and, after 
becoming free from the adjacent cellule, form a slender, gradually cury- 
ing tube, which in mature individuals has its orifice directed backward. 
In Dendrograptus, the form of the orifice and outline of the aperture 
present variations similar to those of the simple uniserrate graptolites ; 
but some species show modifications in the form of the cellule which do not 
accord with the more simple forms. In Dictyonema, the cellules are not 
fully known ; the orifices are marked by a prominent mucronate extension, 
and apparently simulate the more common forms of graptolites. (Fig. 5, 
plate B.) 
