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PROCEEDINGS OF THE 



[July, 1857. 



mals now actually existing, and in the second, especially impro- 

 bable that they should belong to communities of animals such as 

 the dendritic Bryozoa and Hydroids. In the first place both the 

 Halcynoidea, the Hydroidea and the Bryozoa, were all abundantly 

 represented at the present day. If, therefore, the Graptolites are 

 their fossil representatives, why are thej'' cou fined to the Palseozoic 

 period. Were the slates of the Palaeozoic period more peculiarly 

 adapted to their preservation than the Lithographic Rock of 

 Solenhofen, belonging to the Secondary period, or than the Chalk, 

 or than the Marls of the tertiary period. In fact it appears from 

 our present knowledge of the Geological ages, that the Graptolites 

 did not outlive the period in which their fragmentary skeletons are 

 found ; that to that period they belonged, and that they have 

 never had representatives among mature animals since. 



In the second place the Graptolites, though consisting of ser- 

 rate stems, like many Hydroidea and Bryozoa, yet have their 

 stems of a different form, i. e. oflenest entirely unbranched, and 

 without root-like processes, and when the former are present, 

 as in Didymograpsus, they are simple divarications of the 

 extremity of a main stem, and when the roots appeared they 

 were very short and opposite, and belonging to a form of Grapto- 

 lite which was more easily explicable on the hypothesis about to 

 be presented. In short, all the cases of branches and roots which 

 had been figured, with, perhaps, one or two exceptions, such as 

 Hall's fig. 6a, 6c, pi. 74, (Palaeontology of New York, Vol. 1,) 

 were he thought more explicable on the supposition, he would 

 present than by a comparison with Hydroids or Bryozoa, or 

 polypidomata of any sort. 



There was one structure also which imparted a peculiar 

 appearance to the outline of some Graptolites which had so far as 

 was known to Prof. McCrady, no parallel among polypidomata; 

 it was the large, smooth, sharp, thorn-like process which some- 

 times singly issued from one side of a Graptolite, and which had 

 no analogy with a root. Such a process was common in Didymo- 

 grapsus on the outer side of the point of union of the two branches. 

 It was also present in cases where the branches formed an angle 

 with each other almost equalling 180° and therefore lying 

 nearly in the same straight line. Such an instance was repre- 

 sented by Hall, pi. 74, fig. 5a, 55, vol. 1. Also in Pictefs PaU- 

 ontologei, Atlas, pi. cviii. fig. 22, 23. 



