(JRAPTOLITES OF NEW YORK, PART 1 <>23 



apertural margins are also concave and foim an angle of 80° Math the axis 

 of the branch. 



Position and localities. G. thureauiis common in graptolite bed 2 

 (Tetragraptus zone), but extends apparently in diminished size and develop- 

 ment, into the next zone, that with D i d y m o g r a p t u s b i f i d u s . In 

 Canada it is according to Ami's lists restricted to one locality at Levis, 

 where it is associated A\dth the Tetragraptus fauna. McCoy recorded the 

 originals as occurring in the " Llandeilo " of the Bendigo gold field of 

 Australia. It is hoAvever certain that it there also occurs at about the 

 same horizon in rocks of Arenig age. The significance of the distribution of 

 this form for the reconstruction of the paleozoic marine basins has 

 been discussed in the chapter on the range and distribution of the 

 gi'aptolites [p.503]. 



Memarlcs. The dimensions and angles, as well as the general structure 

 of the Deep kill specimens, agree closely with those given in the descrip- 

 tions of the Australian and Canadian types, leaving no doubt of the 

 specific identity of the forms. At the same time the Australian specimens 

 are reported to have 40 branches, the Canadian types even attain as many 

 as 80, "while none from the Deep kill have been observed to have 

 developed, even in large specimens, more than 24, or six on each stem. 

 Though the fact is now ^vell established that, in the multiramous 

 dichograptids, the number of branches furnishes no criterion for generic dis- 

 tinctions, it is also proved that in the phylogeny of this group there can 

 be recognized a tendency to reduce the number of branches and to attain 

 a fixed, though restricted number. In this process the Deep kill forms 

 seem to have reached a stage beyond that of the more multiramous 

 Canadian and Australian forms. The "writer proposed in a former paper 

 [1902, p.580] to designate this stage by the varietal name postremus. 



In the paper mentioned it has been demonstrated by means of the 

 numerous growth stages of this species found at the Deep kill, which 

 furnish an unbroken series [pi. 6, fig.1-10 of this memoir], that the "funicle" 

 consists of two thecae, formed by dichotomous branching from the sicula, 



