STUDY OF THE GRAPTOLITES. 



199 



and with double ranges of cellules, the mucronate or setiform extensions 

 are usually from the lower extremity of the cell-aperture, as illustrated 

 in fig. 31. 



In all those forms of which G. bicornis may be regarded as the type, 

 these processes, when existing, are extensions of the test above the aper- 

 ture, so far as observed (Plate ii, figs 1, and 9) ; or as in species of the 

 character of fig. 20, Plate ii. 



In some species of Diplograptus there is a single mucronate extension 

 from the lower side of the cell aperture, as in the accompanyning illustra- 

 tion, fig. 31. 



Fig. 31. 



GaAPTOLiTHus wHiTFiELDi: twicG enlarged. 



In others, as G. quadrimucronatus (Plate iii, figs. 1 and 2), there is a 

 mucronate point extending from each of the lower lateral angles of the 

 cellules ; as also in G. testis of Barrande ; except that in the Canadian 

 species these appendages are more rigid. 



In Phyllograytus typus and P. ilicifolius^ these processes are apparently 

 the extension of the angles of the cell-partition. 



The cellules of Dendrograptus^ Callograptus and Dictyonema sometimes 

 show mucronate extensions from their outer margins. In Retiolites the 

 cellules sometimes terminate in a plain margin, and in one species the 

 divisions are extended in short strong mucronate points (Plate i, figs. 5 

 and 21, and Plate iv, fig. 11). 



All the species of Retiograptus have the margins of the stipes gar- 

 nished with slender mucronate points, corresponding to the cellules, and 

 extending almost rectangularly to the axis (Plate iv, figs. 8 and 9). 



