1*76 Hall on the Genus Graptolithus. 



branches. One specimen preserves a thick corneous substance, 

 which is the exterior surface, while the other preserves the mould 

 of the opposite side, the radiating impressions of which are cre- 

 nulated. There are no evidences of branches extending beyond 

 the margin of the disk. 



We have now so many well-established forms in the family 

 Graptolitidece, that we have the means of comparison with other 

 allied families among palaeozoic fossils. 



Although numerous species in this collection are shown to be 

 of compound structure, or to consist of fronds composed of two or 

 more branches, and many of them originating in, or proceeding 

 from a disk of thickened corneous substance, yet it is not impro- 

 bable that there are among true Graptolites simple stipes or stems, 

 as all the species have been usually heretofore regarded. I am 

 disposed to believe that those Graptolites where the stipe is ser- 

 rated on the two sides {Diplo'/rapsus) may have been simple from 

 the base ; and that the branching forms having both sides, or one 

 side only of the branches serrated, may possibly also have been 

 simple, or bearing no more than a single stipe from the radicle. 

 The bifurcate appearance at the base of G. bicornis however, 

 offers some objections to this view, and these too may have been 

 compound, like those which have only one side serrated. 



The numerous compound forms shown in this collection, and 

 the great variety of combination in the mode of branching, in- 

 duces the belief that all those with a single series of serratures 

 have been originally composed of two, four, or more branches, 

 either diverging from a radicle or connected by a vinculum from 

 which the radicle has extended. 



The Phyllograptus, although apparently an anomalous form, 

 is not more so with our present knowledge of the Graptolites than 

 G. Logani or G. octobrachiatus would have been considered a few 

 years since. 



It is not among the least interesting facts, that we should find 

 the Graptolitidece simulating in their mode of growth so many of 

 the Palseozotic Bryozoa. We have Fenestetla represented in 

 Dictyonema ; the ramose forms of Retepora in Dendrograptus ; 

 Glauconome and Ichthyorachis in Plumalina ; while the spirally 

 ascending forms figured by Barrande appear to simulate in their 

 mode of growth the spiral forms of Fenestella or Archimedes. 



The forms of Graptolites now known are so numerous as to de- 

 serve especial consideration in their relations to other groups or 



