cxxxvi BRITISH GRAPTOLITES. 



pendent forms attached to floating sea-weed, this generalisation harmonises the 

 previously known facts as respects their special mode of occurrence, their universal 

 dissemination, their superabundance in carbonaceous deposits, their restricted 

 geological range, and their broad lines of evolution in time. 

 1897. 



Freeh, In 1897 Freeh published an extended monograph on the 



" Lethsea Geognostica," Graptolites in general, in his continuation of Roemer's great 



in continuation of work « LefcliaBa Palgeozoica,' which was left unfinished at his 



Roemer's ' Lethsea -. . , 



„ , . , , . death. 



.Palseozoica, vol. 1. 



In addition to epitomising and illustrating the discoveries and conclusions of 

 previous observers, Freeh made many new theoretical suggestions, especially as 

 regards the classification of the Graptolites. 



Organisation of the Graptolites. 



Freeh commences with discussing the organisation of the Graptolites, dealing 

 first with : 



A. Organisation of the Fully-grown Animal. — Broadly speaking, he adopts the 

 views of Ruedemann with relation to the so-called pneumatophores, gonangia, etc., 

 and extends them to all the Diplograptida?. In the Dendroidea, however, he 

 regards the gonothecee of WIman and Holm as corresponding to the nematophores 

 of the Hydrozoa. 



As regards the structure of the test, Freeh does not adopt Perner's view of the 

 existence of a fourth layer. 



Freeh lays great stress on the free-swimming or floating character of the 

 Rhabdophora, and explains many of the peculiar structures found in Graptolites 

 by the assumption that they were connected with their swimming mode of life. 

 He recognises four different modifications of swimming organs : 



1. The bladder in Diplographis physophora is a " rudder-like propelling organ." 

 The same is the case with the vesicle in Monograptus pala, of which he gives a 

 theoretical drawing of colonies attached to a float. 2. The so-called disc at the 

 base of Glimacographis bicornis he regards as having served in some way for the 

 movement of the animal. 3. A third modification is found in Gephalograptus, 

 where the " whole surface of the hydrothecas has widened and taken on a rudder- 

 like form." He gives a theoretical drawing of Petalogr aphis folium attached to a 

 float. 4. A fourth modification . occurs in Dicellogr aphis divaricatus, in which a 

 membrane exists between the branches. 



All these aided in giving the Plankton colony-animals an undulating up-and- 

 down movement rather than a forward one. As it is doubtful whether all the 

 Monograph possessed floats, the " float," therefore, must not be regarded as an 

 organ of systematic importance. In those forms that have a float, " the axis 



