468 



Proceedings of the 



Branches are given off at irregular distances from these prin- 

 cipal stems. The length of the polypidom. has been very 

 great ; one specimen I have been able to trace for nearly three 

 feet. The polypidom has been formed of a flexible substance; 

 for they are seldom found in straight lines, but generally in 

 curves, or bent, without breaking, on themselves. The ap- 

 pearance of this zoophyte 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 continua- 

 tion of the line of the axis ; it is 

 shorter than the lateral ones. 

 They generally 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 slender than in 

 V.foliaceus, which in general out- 

 line it somewhat resembles. The 

 axis is slender, and produced be- 

 yond the other parts of the fossil. 

 The cell-walls are well marked, extending upwards from the 



+ 



