PLATE III— Continued. 



Graptolithus bigsbti, Hall. 



These figures are given to illustrate the phases assumed by a four-stiped species (Tetragkaptus) in its different 

 positions and manner of imbedding in the shale. {Seepage 2il.) 



Fig. 18. Shows a close arrangement of the parts and the contiguity of the non-ceUuliferous edges 

 at the apices, which are scarcely separated in the shale. 



" 19. Illustrates a common condition of this species, where two of the divisions show the 

 lateral faces, while the non-celluliferous edge of a third division is seen lying nearly 

 vertical in relation to these. The fourth division haS been broken off in the sepa- 

 rated film of slate. 



" 20. A specimen showing the lateral faces of two divisions. Below these, in the shale, are 

 seen the non-celluliferous edges of the two other divisions. 



" 21 . An enlargement of an individual where the divisions are equally spreading ; one of them 

 preserving only the base of the stipe. 



Graptolitmvs octonarius. Hall. 



Fig. 22. An enlargement of a specimen much broken and distorted from pressure, showing the 

 mode of bifurcation in this type, where there is no central disc. 



Graptolithus (LoGANOGKAPTus) octobrachiatus, Hall. 



Pig. 23. The central disc and bases of the stipes of a large individual of this species. The 

 original preserves two of the stipes to the length of eight inches, and another to 

 nearly the same extent, while the rest are broken oiF at less distances from the disc. 

 The figure shows that the bifurcations of the stipes take place within the limits of 

 the disc, and become celluliferous just beyond its margin; also that the disc is com- 

 posed of two separable layers of substance, enclosing the bases of the stipes between 

 them. 



" 24. The extension of one of the stipes of the specimen, fig. 23. 



" 25. An enlargement from the stipe, fig. 24, where the substance is obliquely compressed at a. 



" 26. An enlargement from the same stipe at the point where the substance is laterally com- 

 pressed at 6. 



" 27. An enlargement from the same stipe at the lower point, looking upon the apertures of 

 the cellules, which are somewhat compressed. 



Graptolithus flbxilis, Hall. 



Fig. 28. A portion of a branchlet enlarged, showing one part compressed laterally, with the 

 cellules fully expanded ; while the other, on the right hand, is gradually twisted so 

 as to show only the back of the branchlet. 

 " 29. A bifurcated fragment enlarged ; the cellules have been flattened vertically, causing them 

 to be visible in slight indentations on both sides of the axis, giving it the appearance 

 of a DiPLOGRAPTUs. This enlargement is made from a part nearer the centre of the 

 same specimen as fig. 28. 



Graptolithus richardsoni, Hall. 



Fig. 30. The figure shows one of the main stipes of a properly branching species ; the original 

 specimen having all the characters of a monoprionidian form of Graptolite, where 

 the stipes are branching and again dichotomous throughout their entire length (== 



DiCHOGRAPTUS, SaLTEr). 



