from the Silurian Shales of Dumfriesshire. 



25 



phyte in its living state — its long graceful stem yielding to the 

 motion of the water, and its crown of tentacles occupying every 

 cell, which were sufficiently removed to exhibit their individuality 

 — must have been very beautiful. 



The polypidom at its origin, near to the slender base, is very 

 narrow, being little more than a fine line ; as it increases in 

 length it increases in breadth, until it is fully two-fifths of a 

 line broad. The cells are very remote from each other, and are, 

 at first sight, from the slight indentation they make in the stem, 

 scarcely perceptible, giving the Graptolite the appearance as if it 

 were a clear line. The mouth of the cell is straight and at right 

 angles to the axis; it makes an indentation equal to about one- 

 sixth of the breadth of the polypidom. The number of cells in 

 an inch is about eighteen. 



Diplograpsus tricornis. 



This species can be readily distinguished by the three spines 

 which adorn its base, and which are almost always preserved. 

 The central spine is a continuation 

 of the line of the axis ; it is shorter 

 than the lateral ones. They gene- 

 rally form a more or less acute 

 angle with it, and are never farther 

 removed than to form a right angle ; 

 occasionally they assume a graceful 

 curve. The polypidom is more slen- 

 der than in D. foliaceus, which in 

 general outline it somewhat resem- 

 bles. The axis is slender, and pro- 

 duced beyond the other parts of 

 the fossil. The cell-walls are well 

 marked, extending upwards from 

 the axis to the boundaiy of the 

 fossil. Each cell forms a rhomb 

 whose outer border is slightly indented, gi\ang the boundary of 

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

 so as to show the serratures, the spines are so compressed that 

 the central one is almost or altogether lost. When the spines 

 are well preserved and in the position described, 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. 



* 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. 



i 



