56 



Mr. J. Hopkinson on the 



the sections of the perisarc and line of junction of the thecse 

 with it could not be thus explained. 



In the other specimens examined very similar appearances 

 are presented. In both Didymograjytus extensus and D. 

 patulus there is a jointed perisarc with thecas distinctly sepa- 

 rated from it. They appear, in fact, to have budded from 

 it as the leaves of an exogenous tree bud from the stem or 

 twig- which supports them, and not to have been continuous 

 with it as are the leaves of endogenous trees with their sup- 

 port. In this point it would seem that we have an analogy 

 with the vegetable kingdom. It is well known that in some 

 graptolites we frequently have the perisarc preserved without 

 the calycles which should spring from it ; and so may we 

 have a tree without its leaves, though in both cases there has 

 been organic connexion between the now disconnected mem- 

 bers. In the graptolite, as in the tree, there is no actual 

 septum ; there is a ridge, a constriction, occasionally forming 

 a very sharp line of demarcation, but in most cases scarcely, 

 if at all, perceptible. 



This " impressed line " I first noticed in 1868 in graptolites 

 from the Lower Silurian rocks of the south of Scotland. In 

 1872 I found several specimens of the species Monograptus 

 hohemicusj Barr., M. Ntlssoni, Barr., and M. leintwardmensis^ 



Figs. 2, 2 a. DifFyrent portious of the branch of Tctragraptus serra 

 represented in fig'. 1, magn. 5 diameters. 



Figs. 3, 3 a. Different portions of a branch of Didymograptus extensm, 

 magn. 5 diameters. 



Hopk., in the Ludlow rocks near Ludlow, more clearly indi- 

 cating such a structure ; but it is not until now that I have 

 been able actually to see not only the external indication of a 



