THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. II. 



No. XII.— DECEMBER, 1895. 



1. — Notes on the Phylogeny of the Graptolites. 

 By Prof. H. A. Nicholson, M.D., D.Sc, and J. E. Marr, M.A., F.R.S. 



SINCE the remarkable paper by Professor Lapworth " On an 

 Improved Classification of the Rhabdophora " was published 

 in the Geological Magazine for 1873, a great deal of fresh in- 

 formation has been gathered as to these interesting fossils ; but 

 the classification given in that paper, though to some extent con- 

 fessedly artificial, is still generally adhered to. Observations made 

 by the authors in recent years lead them to suppose that that classi- 

 fication will in the future undergo considerable modification ; but 

 in the present state of our knowledge it serves a purpose so useful, 

 that it is not our intention to propose any immediate change in it. 

 Our object, on the other hand, is to bring forward certain con- 

 clusions which we have independently reached, and which will, 

 we believe, enhance the value of Graptolites to the stratigraphical 

 geologist, and lead to results important to the biologist. Oar con- 

 clusions are based upon an examination of a large number of forms 

 generally referred to the family Dichograptidai ; but, as we propose 

 very briefly to indicate, they affect the relationships of Graptolites 

 belonging to other families also. 



In general, the number of stipes possessed by Graptolites has 

 been looked upon as a character of prime importance; indeed, many 

 of the genera are based essentially on their possessi(m of a certain 

 number of stipes, as for example, Lognnograptus, Tetragraptus, and 

 Didymograptus. Again, the "angle of divergence" of the stipes 

 lias been considered an important factor in the diagnosis of families. 

 As regards this latter point, however, the existence of a form like 

 Didymograptus gihheridus, Nich., in which the angle of divergence 

 is over 180°, whilst the other characters are those of a true Didymo- 

 grajytus, shows that this angle is not of primary importance in 

 separating the major divisions of the sub-class Graptolitoidea. On 

 the other hand, we have been led to believe that a character of 

 really essential importance in dealing with the classification of the 

 Graptolites, and one which, in all probability, indicates the true 

 line of descent, is to be found in the shape and structure of the 

 hydrothec^. 



In the more ancient types of Graptolites the hydrothecge are 

 comparatively simple, differing little from the initial sicula, except 



DECADE IV. VOL. II. NO. XII. 34 



