their Structure and Affinities, 73 



demand for his opinions the most thoughtful consideration. An 

 interchange of specimens between Europe and America would be a 

 great advantage to observers in both countries ; for while the draw- 

 ings of Barrande, Geinitz, M'Coy, and Salter, convey as accurate an 

 impression of our European forms as can be given on paper, and 

 those of Hall equally so of the American species, yet it is impossible 

 for students fully to understand the nature of the organisms without 

 their careful examination ; we must always on this side of the Atlantic 

 have a defective knowledge of Bichograpsus, and the new genera of 

 Hall, until we have an extensive series for examination ; and it is to 

 be expected that on the other side erroneous ideas will be enter- 

 tained regarding our old-world species, from the same cause. Had 

 Hall examined perfect European specimens of the genus Graptolithus 

 he would have seen that very few of these could be parts of the re- 

 markable compound forms which he so admirably describes and illus- 

 trates in the last Decade of the Canadian Survey. 



In 1857, Hall established the genus Phyllogr&ptus. In subse- 

 quently published papers he proposed several additional genera, 

 some of which are probably not true graptolites. In some he can 

 detect no cell-openings, and hiocaulis, which is not rare in our 

 British Silurians, has a solid homogeneous structure very different 

 from that of any graptolite with which I am acquainted. Dendro- 

 graptus is an interesting form represented in Britain, and his genera 

 CUmacograptus and Dicranograptus, established for forms already 

 known, are well characterised. 



This closes the history of the different genera of British graptolites. 

 I shall now give an analytical key to the genera, and proceed to the 

 enumeration of the species, having occupied much more space than 

 I intended with this introduction. 



ANALYTICAL KEY TO THE GENERA. 



A. Polypary with a single series of cells. 



a. Polypary simple. 



a. Cells free throughout their whole length. Hastrites, Barr. 



b. Cells in contact throughout more or less of 



their length. Graptolithus (Linn.), M'Coy. 



b. Polypary compound. 



a. Polypary growing in one direction from the 



primary point, Cyrtograpsus, Can*. 



h. Polypary growing bilaterally and consisting 



of two simple or double branches, Bidymograpsus^W Coy 



c. Polypary growing bilaterally and branching 



regularly, and furnished with a central 



corneous disc. Bichograpsus, Salter. 



d. Polypary growing bilaterally, irregularly and 



repeatedly branching and rebranching, and 



without a central disc. Cladograpsus, Carr. 



e. Polypary with a thick common hydrocaulus, 



and branching irregularly. Bendrograptus, Hall. 



B. Polypary with two series of cells. 



a. Polypary with a slender solid axis, 



a. Cells which are true hydrothecse, Biplograpsus^ M'Coy, 



b. Cells hollowed out of the common periderm. CUmacograptus, Hall. 



b. Polypary without an axis. Retiolites, Barr. 



C. Polypary with single and double series of cells. Bicranograptus, Hall. 



D. Polypary with four series of cells. Fhyllograptus, Hall. 



