STUDY OF THE GRAPTOLITES. 



213 



Fig. 22. 



diprionidian groups; though the typical forms, Grai^tolitlms {Diphgniftus) 

 pristis (fig. 22) and allied species, never show any tendency to a division 

 of the parts of the stipe ; and we shall observe, as we 

 progress, that these forms are connected with other 

 diffisrences of structure. 



The types of this specie (Diprion or Diplograptus) are 

 simple linear or sub-linear stipes, bearing a range of 

 cellules on each side, often showing an initial point or 

 radicle at the base, and an extension of the slender axis 

 above and beyond the celluliferous portion. This axis is 

 sometimes divided, showing it to be composed of two 

 distinct laminjB, but this condition is extremely rare 

 among American specimens. 



From these forms with a double series of cellules, we 

 pass to the broad filiform stipes, which are apparently 

 composed of four semi-elliptical parts conjoined along 

 their straight sides, and thus present four ranges of 

 cellules. These cforms (Phyllograptus) are, in some 

 species, broad and short, while others are elongate, with sub-parallel sides, 

 as shown in figures 23 and 24 of Plii/llograptiis angiistifoUus and P. typus, 



and it may not always be possi- 

 ble to distinguish them from 

 Diplograptus. 



From the occurrence of a large 

 number of these leaf-like stipes, 

 sometimes crowded together in 

 a small space, I have inferred 

 that they may have grown in 

 the same manner as Retiograp- 

 Tus (Plate iv). 



Following the forms with a 

 double series of cellules, are those 

 of similar general form, the Re- 

 TiOLiTES, which are known only as simple stipes (Plate i,figs. 20 and 21). 

 In Retiograptus, the separate parts consist of simple elongate 

 stipes with the axis projecting at either extremity ; and in one form 

 (Plate iv, fig. 9), we have the stipes connected by slender basal exten- 

 sions of the axis, and the whole united in a spreading frond, in a manner 



Fig. 24. 



