246 NATURAL SCIENCE. October. 



of branching in DendvcgrapUis (?) balticus, Dendrograptus (?) hottnicus, and 

 Didyouema peltahmi. It seems, therefore, to be fairly common. There 

 are, however, some variations. For instance, in Dictyonema peltatum, 

 of the individuals already developed in the mother branch, an old 

 theca passes up into the one branch, and into the other an old 

 theca and an old gonangium. 



I now proceed to the description of a dendroid graptolite o 



somewhat different structure, Ptilograpius suecicus, Wiman, Fig. i8. 



Here at the outset we have to distinguish between twigs and 



branches. The branches carry twigs springing out feather-fashion 



alternately to right and left, and these consist of 



four individuals, opening one after the other. The 



first opens on the back, in the angle between the 



branch and the twig, and therefore is not visible on 



Fig. 1 8. The second is seen opening on Fig. i8. 



The third again opens on the back of the twig, at 



about the same height as the second. The fourth 



Fig. i8.— Branch jg terminal. On the branches, which together give 



OF Ptilograpius ^^^ rhabdosome a bush-like growth, no individuals 



suecicus. , , , 1 ,, , 1 



J . open, but on the other hand they are all placed 



2 and 4, openings ^ ' j t- 



of second and fourth immediately on these. In Fig. 19, section A, are 

 individuals in the seen four individuals, a, b, c, and i, which will 

 proximal twigs of open in connection with a left-hand branch. The 

 e rig an e individuals rtj, Z»i, c^, and 2 open with the next right- 

 hand branch. Of individuals belonging to the next 

 left-hand branch only bn is as yet ready ; <^ is a budding individual, 

 and such never open outwards. 



As sections A and N are the reversed images of each other, 

 we can make the series as long as we want. Towards the proximal 

 end we may prolong the series from A backwards, with the reversed 

 images of sections N, M, L, etc., and towards the distal end we 

 may prolong it from N onwards, with the reversed images of sections 

 A, B, C, etc. If we now, to begin with, follow the series backwards 

 to the reversed image of section N, we find that the individuals Ci, 

 2 and d in A are three young brother individuals, of which d, as is 

 shown by section F, is a budding individual, while 2 is a theca, 

 and Ci is a gonangium. Frequent repetition of the same operation 

 shows that the individuals belong together in generations in the 

 following way. In the table I begin with the oldest generations : — 



Budding Individual. 



d 



dn 



From this table the age of each individual is at once seen. The 



