456 Br. 0. Herrmann — Distribution of Graptolifes. 



however, be indicated, that the branches of Clonograptiis multiplex, 

 Nich., and C. flexilis, Hall, have cells between the furcation-points. 

 Finally, T will mention that a specimen of Tetragraptus friicticosns, 

 Hall, collected by myself, distinctly shows a hydrotheca between the 

 sicula and each of the two furcation-points. T. fructicosus, Hall, 

 as is well known, has a mode of ramification different from that 

 of the other Tetragrapti, for which reason one more readily manages 

 to see the hydrothecse in profile. The branch-segments situated 

 between the sicula and the furcation-points are, however, here the 

 same parts of the hydrosoma which are denominated the "funiculus" 

 in the other Tetragrapti, and this part is sometimes celluliferous in 

 T. fructicosus, Hall. 



The facts brought together above show that the exceptions to Hall's 

 rule are gradually accumulating. We begin to feel the necessity of 

 dropping the definition of the funiculus as a cell-less part of the 

 hydrosoma ; but in any case the presence of such a naked stem is 

 not to be employed as a means of separating genera and species from 

 each other. The species founded upon such a criterion runs the risk 

 of being upset by every fresh observation. 



The Central Disc. — The most singular of all the parts of the 

 Graptolite-body is undoubtedly the central disc, i.e. a chitinous mem- 

 brane, which, in some much-branched Dichograptidee, is extended 

 between the basal parts of the branches. This peculiar organ was 

 discovered by Hall, and gradually detected upon five different 

 American species of the family DichograptidEe, Lapw. Some of the 

 American disc-bearing species were also soon found in the English 

 Silurian deposits. 



For the paleeontologists upon the continent of Europe the descrip- 

 tion of this singular appendage had a peculiar interest, as it could 

 not but be supposed that similar interesting forms would be dis- 

 coverable in the strata which were regarded as of the same age as 

 these American and English deposits. That these anticipations were 

 founded upon a correct assumption was shown by the discovery of 

 a comparatively thin but most productive bed of the expected and 

 wished for forms. Those of this locality (Galgenberg in Oslo, 

 a suburb of Christiania; Lower Graptolite- shales of Kjerulf; Phyllo- 

 graptiis-shales of Brogger) contributed to the completion of the 

 analogy between the Norwegian, American, and English strata in 

 question. 



The number of species of Dichograptidse with a disc has now been 

 raised to seven, the names of which here follow : Tetragraptus alatus, 

 Hall, T. crucifer. Hall, T. ITeadi, Hall, T. approximatus, Nich., Dicho- 

 graptus octobrachiatus, Hall, D. Kjerulfi, Herrm., D. (Loganograptus) 

 Logani, Hall. In specimens of Climacograptus bicornis, Hall, also, 

 a similar membrane is sometimes observed, enveloping the base of 

 the celluliferous stem and the processes issuing from the stem ; and 

 in some species of the genus Dicellograptus, Hopk. (e.g. D. Moffatensis, 

 Carr.), the proximal parts of the two branches are also united by 

 a similar membrane. 



Hall, to whom we are indebted for all the information relating 



