cxxx BRITISH GRAPTOLITBS. 



layer of the rhabdosome wall, but a peculiar calcite deposit formed during the 

 process of fossilisation, the formation of which was only possible when the skeleton 

 of the rhabdosome wall — namely the black layer — was surrounded by an organic 

 integument." 



The " palisade " layer (layer with colonnettes) he also regards as another form 

 of the calcite deposit surrounding the black layer. Giirich concludes from his 

 investigations that the chitinous skeleton of the rhabdosome in the living condition 

 was surrounded by a skin, but that it is impossible to say of how many layers 

 this skin consisted, or what were their particular histological peculiarities. The 

 existence of such an outer skin is proved by the presence of growth-lines, and the 

 chitinous skeleton, instead of being external, is mesodermal. These conclusions of 

 Giirich are in accordance with Wiman's views (' Ueber die Graptolithen,' 1895). 



The form of the aperture in Monograptiis priodon is dealt with, and Giirich 

 disputes Jaekel's idea of a laterally expanded projection, pointing out that the 

 theca in this form is merely a " tube whose open oval end is bent back towards 

 the sicula." He gives a figure showing a schematic reconstruction of the cells of 

 this species. 



The paper concludes with remarks on the mode of life of the Monograptidse. 

 He considers that it is very improbable that they were attached to the sea floor, 

 and argues that their geological distribution, their method of preservation, etc., 

 speak in favour of their being Plankton. 



Ptuedemann's discovery of colonies of Diplograptus justifies us, he considers, in 

 concluding that the Monograptida? possessed a swimming bladder, and also that a 

 large number arose from one and the same stock. In this case the relationship 

 between the rhabdosome, sicula and disc becomes of primary importance, the form 

 of the aperture is secondary, and is the result of such a relationship. He re- 

 figures and discusses his schematic representation of the differences between the 

 Monograptus erecti and M. reversi groups. 



ng 9 g In 189G Gurley published a paper entitled "North 



GurJey, American Graptolites," in which he gives a complete list of 



" North American American forms, discusses the synonymy of many of the 



G-raptolites, ' ' Journ. genera and species, and describes a large number of new 

 Geol.,' vol. iv, no. 1. . . .-, -, n • xi 



species, without, however, figuring them. 



Description of Species. — The following forms are referred to or described : 

 Phyllograptus ? cambrensis, Bryograptus? multiramosus, Dichograpsus remotus, I), 

 abnormis, Tetragrapsus acanthanotus, Didymograpsus bipunctatus, D. perflexus, 

 D. geminus, D. hirundo, D. convexus and D. sagitticaulis. 



The generic name Stephanogra/ptus should, he considers, take precedence of 

 Helicograptus and Goenograptus : two new species are described : 8. crassiusculus 

 and S. exilis. Azygograptus is represented doubtfully by one species A. ? Walcotti. 



