292 NEW YORK STATE MUSEUM 



they have been distinguished into the upper and lower Dicellograptus zones. 

 It is equally well represented in the British Lower Siluric (Ordovician) 

 beds and has been elaborately described in the Monograph of British 

 Graptolites. 



As its most important characters we have to consider the composition 

 of the rhabdosome of two separate main stipes which embrace an angle 

 exceeding 180 and the introversion and final introtortion of the thecal 

 apertures with concomitant excavation of the ventral walls of the succeed- 

 ing thecae. These characters are seen in successive stages of development 

 in different species and therefore lend themselves readily to the distinction 

 of subgeneric groups. 



Since the angle of divergence of the stipes is greater than 180°, the 

 sicula is found inclosed in the smaller angle formed by the dorsal walls of 

 the stipes. This angle, which is here of especial diagnostic importance, has 

 been termed the axillary angle by Hopkinson and the space included 

 between the dorsal walls of the stipes is denominated the axil in the Mono- 

 graph of British Graptolites. The form of the latter is variable and of 

 some phylogenetic importance [see D. smithi and p. 109]. 



The proximal end of the rhabdosome is described as possessing two 

 alternate crossing canals, the same as Leptograptus, and an alternate devel- 

 opment of the four primary thecae. Our material unfortu- 

 nately did not contain any reverse aspects of proximal 

 portions that were distinct enough to show the two crossing 

 gfaptifs 6 ^urViyi canals. Two well preserved obverse views are seen in 



Lapworth. Proximal pur- . . 



tion x 7 figures 206 and 224. They show the point of origin ot the 



first theca, the horizontal direction of the first two thecae and their mesial 

 spines (the apertures being turned upward into the; ventral excavation of 

 the next theca) and the more or less abrupt turning upward of the distal 

 portions of the third and fourth thecae, whereby the stipes are given an 

 ascending direction. 



The form of the thecae, which finds its most distinct expression in the 

 profile view in the direction of the ventral wall and ot the aperture is, as 



