Genus Graptolithus. 143 



the branch upon the exterior surface near the centre, is marked 

 by a depressed longitudinal line, which follows the ramifications 

 and gradually dies out as the branches become finally simple, 

 when the surface on the same side is smooth or somewhat ob- 

 liquely striated. The disk is smooth exteriorly, and from the 

 centre is a small radicle from which the two sets of branches 

 diverge. 



This species, though in a general manner bi-lateral, and present- 

 ing four principal branches, is nevertheless, from the irregular di- 

 vision of these, usually unequal upon the two sides ; and we find 

 on examination of those figured that they are as ten and ten, nine 

 and eleven, eight and nine, ten and eleven, seven and ten, twelve 

 and twelve, eight and eight, eight and ten, while the half which 

 is figured on Plate II. has eleven rays. 



Plate I. Fig. 1. An individual showing the exterior surface ; 

 the central portions entire, with the impression of the connecting 

 corneous membrane, some portions of which remain still attached 

 to the arms. The extent and outline of the membrane are very 

 distinctly preserved. Some of the arms are broken off at the ter- 

 mination of this mQmbrane or disk, while others extend to some 

 distance beyond its limits ; all however are imperfect. 



The appearence of serratures is due to exfoliation, which shows, 

 the impression of the inner side upon the stone. 



Fig. 2. Exterior view of another individual, in which some por- 

 tions of the membrane still remain, the branches being all broken 

 off just beyond the last bifurcation. 



Fig. 3. The inner side showing the commencement of the 

 cells, which appear in some places to be in a double series. 

 The connecting membrane of the branches is removed in this 

 specimen. 



Fig. 4. Enlarged view of the exterior surface of the central por- 

 tion of an individual. 



Fig. 5. Enlarged view of the inner surface, exhibiting the ap- 

 pearance of a double series of cells, separated by a depressed 

 line in the substance of the branch. In some instances these 

 appear to be absolutely separate, while in others they are con- 

 nected, showing that there is but a single series, and the apparent 

 separation is due to the depression along the centre. 



Fig. 6. An enlarged view of a fragment of a branch, showing 

 serratures on one side, with a corresponding row of obscure, ele- 

 vated ridges, which may perhaps be due to the foldings of the 

 branch. 



