538 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [June 17, 



short, their points sometimes slightly turned down. The inferior 

 cell- walls inclined to the axis at from 5° to 10°, and fi'om four to 

 to five times as long as the cell-mouths. 



Like 6r. Sedgwickii, Portl., G. Nilssoni appears under such various 

 forms and aspects that it is extremely difficult to draw exact limits 

 for the species. Taken as a whole the characters of the species are 

 sufficiently distinctive ; but numerous specimens present themselves 

 which approach closely to G. tenuis, Portl., to the young form of G. 

 Sagittarius, Linn., and to certain narrow examples of G. Sedgivickii, 

 Portl., whilst all the different varieties are connected together by a 

 series of transition forms. Three forms, at any rate, differing from 

 one another in size and in some smaller peculiarities, seem to be 

 sufficiently distinct to be considered genuine varieties. Of these, 

 I have taken the one which is intermediate in size to represent the 

 typical form of the species, the two extremes appearing to be truly 

 varieties, and to possess characters really peculiar and not depend- 

 ing simply upon age or mode of preservation. 



The ordinary form of G. Nilssoni, Barr., is characterized by the 

 separation of the cellules, which do not overlap one another, by 

 the short, rectangular, or nearly rectangular, cell-mouths, and by 

 the long and slightly curved "inferior" cell-walls, which give the 

 entire cellule a triangular shape. In G. tenuis, Portl., on the other 

 hand, the cell-mouth, though also short and rectangular, transgresses 

 upon the body of the stipe, running across the base of the cellule 

 immediately above, whilst the inferior cell-wall is either straight 

 or is somewhat convex. The larger variety of G. Nilssoni agrees 

 with the normal form in most respects, differing chiefly in the 

 greater width of the stipe, the more conspicuous common canal, and 

 the fact that the ceU-mouths are dii-ected more upwards, so that 

 the denticles become more acutely angular. The smaller variety is 

 chiefly distinguished by its extreme tenuity, and the proportionate 

 length of the "inferior" cell-wall when compared with the cell- 

 mouth. 



Loc. Coniston Plags of Skelgill Beck, near Ambleside ; Upper 

 Llandeilo rocks of Garple Linn, Duffkinnel Burn, &c. near Moffat. 



Graptolites TENUIS, Portl. PI. XX. fig. 31. 



G. tenuis, Portlock, Geol. Rep. p. 319, pi 19. fig. 7. 



Stipe slender, of unknown length, neither extremity having 

 been hitherto detected, and having a breadth of from -J-q- to -^^ 

 of an inch. The cellules are from 20 to 24 in an inch, overlap- 

 ping one another to an extremely limited extent, very narrow, 

 and inclined to the axis at the very small angle of from 5° to 10°. 

 The cell-mouths are extremely short, at right angles to the axis, 

 and running partially across the stipe, so as to cut across the base 

 of the cellule immediately above. The "inferior" ceU-waUs are 

 straight or slightly convex, from three to four times as long as the 

 cell-mouth, and almost parallel to the axis. 



It is very questionable whether this can be maintained as a dis- 

 tinct species. Xa doubt some of the forms of G. Nilssoni, Barr., 



