EXPLANATIONS OF PLATE III, continued. 

 Graptolithus bigsbyi. 



These figures are given to illustrate the phases assumed by a four stiped species (Tetra- 

 GBAPTUS) in its different positions and manner of imbedding in the shale. See p. 209. 



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

 rous 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 separated 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. 



Graptolithus octonarius. 



Fig. 22. an enlargement of a specimen much broken and distorted from pressure ; 

 showing the mode of bifurcation in this tyi>e, where there is no central disc. 



Graptolithus (Loganograptus) octobrachiatus. 



Fig. 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 off 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 composed of two separable layers of sub- 

 stance 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 com- 

 pressed at a. 



" 20. An enlargement from the same stipe at the point where the substance is laterally 

 compressed at b. 



" 27. An enlargement from the same stipe at a lower point, looking upon the aper- 

 tures of the cellules, which are somewhat compressed. 



Graptolithus flexilis. 



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 

 gi'adually 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. 



Graptolithus richardsoni. 



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 through- 

 out their entire length = Dichograptus. Salter. 



