STUDY OF THE GRAPTOLITES. 197 



In some forms having cellules of this character, as in G. bicornis proper, 

 there is, in the liattened stipes, an external ridge, as if indicating the junc- 

 tion of the axis with the external walls. But in examples where this axis 

 extends beyond the celluliferous portion of the stipe, it is compressed, 

 having the aspect of a flattened cylindrical filiform body. It has no 

 appearance of having been flat, or laterally extended within the body of 

 the stipe. 



In the ordinary forms of graptolites the orifice of communication between 

 the cellules and the common body is usually round, oval, or quadrangular; 

 and this appears to be true of all the species with a single range of cellules, 

 and also of the ordinary forms of those two ranges of cellules where the 

 common body is divided by a longitudinal axis. 



In Retiolites, 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. bico7-nis, 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 borly 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. 7iitidus and G. extensus^ Plate iii, the plane of the orifice is 

 nearly rectangular to the axis of the cellule; while in G. bijidus, G. penna- 

 Udus 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 le.-.s than a right angle with the 

 direction of the geneial axis. It would appear that in the progress of 



on their exterior margins from the upper edge of the orifice, they turn more abruptly 

 towards the axis, while the central portion extends obliquely to the axis, leaving a broad 

 arch above, which gradually becomes angular as it approaches the axis. 



