STUDY OF THE GRAPTOLITES. 195 



ring 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 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 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 

 siquadrangular, becoming more rounded towards the aperture ; and when 

 the upper part of the cellule is free, the aperture is round or elliptical, and 

 in some specimens the calyole is elliptical or cylindrical throughout its 

 entire length. We have examples of the quadrangular cellules in G. 

 extensus (F]site iii, fig. 12) and G. octohrachiatus (Plate iii, figs. 25 and 27) ; 

 as well as in one species oi Phyllograptus (Plate iv, figs. 1, 2, 4 and 5). Where 

 the cellules arc more nearly isolated, they approach more and more to the 

 cylindrical form. As examples of cellules contracted towards the aperture, 

 we have Gra'ptolithus priodon, and G. priodon, Barrande, and G. clinto- 

 nensis, Hall (Plate i, figs. 1*, 2, and 3). 



M. Barrande has remarked that from the circumstance of the partial 

 or complete isolation of the successive alveoles of the same series, we may 

 easily conceive that the walls of contact in contiguous cellules should be 

 double. This fact he has ascertained from decomposing specimens of G. 

 priodon ; and we have the same evidence in some of our species. In the 

 cellules of the ordinary mode of development, each one is an independent 

 part of the organization, and is provided with its individual body and cell- 

 walls, as if each cellule were isolated. Whenever two of these are in 

 contact, the cell-walls coalesce as far as the contact continues ; but when 

 becoming free, the cellule assumes its normal condition. In some specimens 

 of Phyllograptus we find this evidence of double walls in the cell-partitions. 



In G. putillus, illustrated on Plate ii, figs. 10, 11, and 12, the cell- 

 walls, although contiguous to and adjoining the walls of the body (and not 

 free), do not coalesce, but are readily separable without fracturing their 

 substance, and the same is true of G. clintonensis. 



