Royal Physical Society. 469 



axis to the boundary of the fossil. Each cell forms a rhomb 

 whose outer border is slightly indented, giving the boundary 

 of the fossil a faintly serrated aspect. When the fossil is 

 preserved so as to show the serratures, the spines are so com- 

 pressed that the central one is almost or altogether lost. 

 When the spines are well preserved, and in the position de- 

 scribed, no traces of the individual cells are discoverable : the 

 boundary of the fossil is an unbroken line.* 



This species is abundant in a thin bed at Hartfell. 



Didymograpsus Moffatensis. 



This is the most elegant of the British species of this genus. 

 The base terminates in three distinct spinous processes. The 

 zoophyte bifurcates from the m 



base. The general appear- 4 m 



ance is like the figure; or % f 



occasionally the lines form % III nut 



an acute angle for about a Ik 



quarter of an inch, then sud- % 



denly expanding in slight % 



curves, almost at right angles, % 



for a short distance, when it \ J? 



again recurs to its original % If 



direction. The branches are 1 ! 



united for about a quarter of M' 



a line by a slight web, which 



in some specimens is terminated in a fine process of short 

 length, taking the direction of a line bisecting the angle, 

 The cells are arranged on the outer margins ; they are very 

 remote, and penetrate the polypidom to scarcely one-fourth 

 of its breadth ; they form slight openings on the margin of the 

 polypidom, first entering at a right angle, and then suddenly 

 turning downwards. These openings are lengthened ovate 

 pouches, answering exactly in shape and size to the cell-serra- 

 tures of the margin. The number of cells in an inch is about 



* The length of the polypidom is more variable in this Graptolite than in 

 any other I have gathered. A young form, as represented in the figure, is not 

 uncommon. 



