458 Dr. 0. Herrmann — Distribution of Graptolitea. 



"With regard to the function of the disc, Hall supposes that, on 

 the one hand, it gave solidity and support to the basal parts of the 

 branches, and on the other may have served other purposes of the 

 animal economy. Such subsidiary functions of the disc may have 

 been only of quite subordinate nature, seeing that in members of 

 one and the same species the disc may be sometimes present and 

 sometimes entirely absent, and its development commences sometimes 

 in quite young individuals, sometimes in those which are nearly full 

 grown. But as the disc occurs only in those forms in which the 

 branches are remarkably thin in the neighbourhood of the sicula, 

 and as, in consequence of the branches frequently being numei'ous 

 and long, the central part of the organism had no small amount of 

 resistance to furnish, this support may have been the principal if not 

 the sole destination of the disc. This is rendered still more probable 

 because the disc never occurs in other forms which possessed 

 particularly stout branches, and in consequence of their mode of 

 ramification had greater firmness, such as Clonograptus multiplex, 

 Nich., C. Jlexilis, Hall, etc. 



It does not appear to me probable that, as has been suggested, the 

 Graptolites were attached by means of the disc, seeing that in very 

 many specimens the disc is entirely uninjured, and even in those 

 with damaged discs the kind of injury does not indicate their having 

 been torn away, because it is precisely the central part that most 

 rarely shows any damage. 



Hydrothec^. — We are indebted to Hopkinson for a new and 

 important discovery with regard to the intimate structure of the 

 Graptolites.' M'Coy, in 1854, in his "British Palaeozoic Fossils," 

 in the course of the description of a Graptolite, had spoken of 

 " transverse septa," which were to be found at the proximal ends of 

 the hydrothec^, and had also represented the position of these septa 

 in a figure. In 1868, Hopkinson had stated in connexion with this^ 

 that he had been unable to find " any indication of a dividing septum 

 [in Graptolites], if we except a few forms in which there is an 

 impressed line between the hydrothecfe and the periderm." 



More recently Allman, in his " Monograph of the Calyptoblastic 

 or Tubularian Hydroids," compared the hydrothecee of the Ehab- 

 dophora with the sessile nematopbores of the Plumularise, at the 

 same time denying the presence of a septum or constriction, and 

 indicating that later observations had not confirmed the existence of 

 the septa mentioned by M'Coy. 



Hopkinson^ was quite recently in a position to investigate an 

 abundant Graptolitic material from the Skiddaw slates, and he 

 found in it specimens of Didymograptus exienitus, Hall, D. patulus, 

 Hall, and Tetragraptus serra, Brongn. { = T. bryonoides, Hall), which 

 permitted the interior structure to be examined. In some of these 

 specimens he was able to perceive that the hydrothecas were separated 

 from the coenosarc by a well-marked septum, and that the ccenosarc 



1 Ann. Mag. Nat. Hist. Jan. 1882, pp. 54-57. 



" Journ. Quek. Micr. Club, vol. i. p. 161. 



3 Ann. Mag. Nat. Hist. Soc. ser. 5, vol. ix. (1882), pp. 54-57. 



