GKAITOLITES OF XEW VOUK, I'AHT 1 



685 



Position and locality. In the shales with Diplograptus dentatus 

 at Mt Moreno near Hudson. 



Remarhs. This species is nearest related to D . a c u t i d e n s Lap- 

 worth, of -which it presents the characteristic 

 features in a I'urther developed stage. As it also 

 succeeds the latter in geologic time, it may well 

 be considered as a descendant of it. It differs 

 from D . a c u t i d e n s naost markedly in the prominence of the apertural 

 portion of the thecae, which in most thecae appeal's cuspidate instead of 

 acutely dentate, as in D . a c u t i d e n s . This is due to a widening, often 

 somewhat abrupt, of the thecae in their last growth stage. 



Fig. 811 Did y mo jr rapt us cuspi- 

 dal us sp. no\ . Kninmeiit of branch. 

 Ashhill quarry at Mt Moreno, .y.-j 



Didymograptus nicholsoni Lapworth 

 var. planus EUes <fe Wood 



Plate 13, tigiires 10-14 



Didymograptus nicholsoni var. planus Elles & "Wood. Monogr. Brit. 

 Grapt. ptl. Pal. Soc. vol. for 1901. p.29, pl.2, iig.5a, b 



Description. Primary disk and nema unknown. Sicula small (about 



1.6 mm long), relatively broad. Branches straight, or very slightly curved, of 



rigid appearance, both together forming a straight 

 line (angle of divergence 180°) ; short (maximal 

 length observed 28 mm) ; narrow, of nearly uniform 

 width (about 1.1 mm) after the sixth theca (width 

 in proximal part .5 mm). Thecae numbering 10 to 

 11 in 10 mm, narrow, four times as long as wide, 



inclined at 20", in contact 



for but one fourth to 



one third of their length. 



Outer margins straight. 



Apertural margins slightly concave, normal on axis of theca ; two thirds the 



width of the branch. 



FiK. 81 Didymograptus 

 nlcnolsoni Lapw. rar. pla- 

 nus E. & W. Proximal portion 

 of rhabdosome. Obverse view. 

 Deep kill. x~ 



Fig. 82 Didymograptus nicholsoni 

 E. & W. Fragment of branch. Deep kill. .x7 



Lapw. var. planus 



