322 NEW YORK STATE MUSEUM 



and the eastern and Arkansas specimens did not fail to show an}- decided 

 approximation to the condition found in whitianus. It seems to us, 

 however, that there exists considerable variation as to the extension of the 

 spines on the thecae between the various rhabdosomes. In some [see text 

 fig. 248] the spines do not go beyond the bifurcation, in others [text fig. 

 247] they extend beyond it by three or four thecae, while in others again 

 [text fig. 246] they extend as far as the branch is preserved. It can, there- 

 fore, be well conceived that there exists a continuous series of gradations, 

 to forms with the spines extending over the whole branch as in D. n i c h - 

 o 1 s o n i var. w h i t i a n u s. 



Dicranograptus nicholsoni var. diapason Gurley 



Plate 21, figures 3-5 



Dicranograptus nicholsoni var. diapason Gurley. Jour. Geol. 1896. 4:73 



The original description of this striking variety is : 



Proximal portion with three minute spines at base, measuring from 

 base to notch between branches five (sometimes as much as 6 mm), showing 



250 



252 



Fig. 249-54 Dicranograptus nicholsoni var. diapason Gurley. Fig. 249-51 Gurley's original (manuscript) 

 drawings (all natural size). Fig. 252-54 Enlargements (\ 5) of portions of specimens from Glenmont, N. V. Originals 



of figures 249 to 251 in National Museum 



below the level of the notch, at most seven (usually six, sometimes five) 

 thecae; width of proximal portion 1.25 to 1.50 mm; branches, in the com- 

 pressed condition, 1 to 1.25 mm wide; diverging at an angle of 45" (some- 

 times slightly less) to 50", with a very gentle inward curve which brings 



