Dr. 0. Herrmann — Distribution of Orai^toUtes. 455 



four, eiglit, or more branches, unites the two symmetrical halves, as 

 well as to the parts of the branches situated nearest to the furcation- 

 points. In the four-branched species the uniting transverse beam is 

 said to be always destitute of cells ; in those with eight and more 

 bi'anches neither the central part of the hydrosoma, nor the portions 

 of the branches placed between the divisional points, are said to be 

 cell-bearing, but the cell-bearing parts or " true branches " only 

 commence beyond the last furcation-point. Hall himself, however, 

 furnishes an exception to the rule thus established. The above 

 remarks do not apply to Clonograptus Milesi, Hall, in which the 

 hydrothecse commence immediately beyond the first furcation-point. 



Holm^ states that Hall's assertion has also proved untrustworthy 

 in other forms, and expresses the opinion that " in many cases, 

 although not always, hydrothecse also occur even upon these sup- 

 posed naked spaces." He has actually detected distinct hydrothec^ 

 upon the so-called funiculus in a Norwegian species of ScJiizograptus, 

 Nich. When he had removed the periderm by means of a brush, he 

 noticed the impression of a small hydrotheca on each side of the 

 sicula. 



The instances in which one is enabled to observe the under-surface 

 of the so-called funiculus will be of excessive rarity in consequence 

 of the repeatedly noted peculiar position of the branched specimens. 

 Nevertheless it has appeared to me as if in my young individuals of 

 Tetragraptus quadribracMatiis, Hall, a narrow streak of a hydrotheca 

 peeped out here and there beneath the transverse beam. 



Several specimens in my possession of the species Dichograptus 

 octobracJiiatus, Hall, and D. Kjerulfi, Herrm., are filled up with the 

 rock-mass and have better preserved their original form. At the 

 proximal end of each hydrotheca we observe a constriction on the 

 dorsal surface of the branch which presents itself to the observer, 

 so that the branch appears to consist of nothing but joints inserted 

 one into the other, like the stem of an Equisetum. These depres- 

 sions, each of which therefore corresponds to a hydrotheca, are not, 

 however, found exclusively beyond the last furcation -point, but also 

 here and there within the furcation-points upon the branches, a 

 distinct proof that even the first divisions of the transverse beam, 

 which, according to Hall, belong to the " funiculus," are furnished 

 with hydrothec£e. In the specimens which give rise to the obser- 

 vations just cited, the transverse beam itself unfortunately was 

 not well preserved, so that it could not be decided whether this 

 does or does not bear hydrothecse. It may, however, be assumed 

 with the greatest probability that this is the case here as in many 

 other species. Future observations will bring us certainty in this 

 direction. 



The discovery of these hydrothecse in the vicinity of the sicula is 

 rendered more difficult because at the proximal end of the hydrosoma 

 (as at the distal end) the hydrothecee gradually diminish in size, 

 and are sometimes only of minute dimensions. Further, it may, 



1 " Pterograptus, ett nytt Graptolitslagte," Ofv. Kongl. Vet. Ak. Forh. 1881, 

 No. 4. 



