254 



NEW YORK STATE MUSEUM 



frequent) and in the proximal part diverge horizontally, increasing their 

 divergence immediately after their origin to 2oo"-2 2 5 . The sicula is very 

 short (.4 mm) and inconspicuous; the primary thecae diverge near the 

 aperture of the sicula; their origin and connection have not been made out. 

 The thecae in the initial part of the branches are extremely slender, six 

 times as lomj as wide, 1.6 mm loner, numbering 6 in 10 mm, and hardly 

 overlapping. Their inclination is there but 5" and the branch has therefore 

 a threadlike appearance. In the mature parts the thecae number nine in 

 10 mm, overlap one third their length and possess an inclination of 12 — 15 . 

 The outer margins are straight ; the apertural margin is straight and rect- 

 angular on the axis of the theca. 



■^tJ-^ 13 



Position and localities. The 

 type specimens are from the 

 160 l6: Normanskill shale at Kenwood, 



Fig. 160,161 Didymograptus subtenuis (Hall). Proximal .... 



and distal portions of the type specimen. x 5 where they are associated with 



D. sagitticaulis, Nemagr. gracilis, N. gracilis van surcu- 

 laris, Dicellogr. ramosus, Climacogr. parvus and Lasiogr. 

 mucronatus. We have also found this species in the same horizon at 

 Mt Moreno ; and the collection from Stockport contains a few specimens. 

 It has also been recorded from the rocks of Canada by Lap worth. 



Remarks. Hall had at first identified his types with one of Portlock's 

 species, but, on finding them specifically different, proposed the name sub- 

 tenuis for the American form. Since he has not described the form so 

 that it could be recognized from the description but rather relied on his 

 reference to Portlock's species and several forms occur which though in a 

 general way similar, are generically distinct, it becomes necessary to rede- 

 scribe the species and restrict it to the specimens figured by Hall and which 

 are in the New York State Museum. The latter [pi. 13, tig. 1, 2] show 

 that the species is distinguished from all similar slender associates by the 

 extreme thinness of the proximal portion and the remarkable slenderness of 

 the thecae. In this character as also in the small size of the sicula it 

 reminds one more of D. gracilis Tornquist, an older species of the 



