STUDY OF THE GRAPTOLITES. 227 



mediate between the exterior and the centre, it presents the aspect of 

 fig. 4 ; and when the cutting is carried to the centre, it gives the charac- 

 ters of fig. 5, the cell-divisions apparently reaching to the axis. 



The general form of this stipe in section (fig. 6), approaches that of 

 Retiolites, as shown by Barrande and Geinitz ; and in the arrangement 

 of the common body and axis, there is a departure from the typical 

 diprionidian forms of Graptolithus. In this transverse section we have 

 a somewhat concavo-convex form, which is narrower on the concave side. 

 There is a central or sub-central point indicating the filiform solid axis ; 

 and on each side of this are the divisional cell- walls, which produce a 

 slight contraction of the exterior walls of the stipe at the inner limit of 

 their attachment. Another section (fig. 7), shows the same features, 

 together with the remains of two other cell-divisions, neither of which 

 reach to the exterior walls of the Graptolite ; and the one on the right 

 hand shows the narrow extremity just before joining the axis. 



These sections, together with numerous other longitudinal, transverse 

 and oblique sections, compel us to conclude that this Graptolite possesses 

 a filiform central or sub-central apparently solid axis ; and that the cell- 

 partitions originate from, or are joined to this axis. These cell-partitions 

 appear to consist of triangular plates, which have an unequally arching 

 or convex upper sm-face, and a concave lower surface. This form of cell- 

 partitions would leave the alveoles to communicate at their bases with 

 the common body on each side.* 



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

 there is, in the flattened stipes, an external ridge, as if indicating the 

 junction of the axis with the external walls. But in examjDles where 

 this axis extends beyond the celluliferous portion of the stipe, it is com- 

 pressed, 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. 



* The cell-partitions in this form of Graptolites are represented as they appear to exist in the 

 solid specimens examined, on Plate ii, fig. 9, where, curving gently downwards on their exterior 

 margins from the upper edge of the orifice, they turn more ahruptly 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. 



