692 NEW YORK STATE MUSEUM 



The same specimen throws also some light on a peculiar feature which is 

 most noticeable in D . b i f i d u s, namely the presence of small pustules at the 

 bases of the thecal walls. These were also noticed by Hall.^ They are not 

 always present ; hence an accidental feature. When present they are situated 

 with great regularity at the place mentioned. This fact and their composition 

 of iron pyrite indicate that they originate by crystallization of j)yi"ite in the 

 basal rings, which, as Perner's sections of specimens of Monograptus [loc. cit. 

 pi. 1, his renjlements piriformes a Vextreniite des ouvertures internes] show, were 

 formed by the pyrif orm swelling of the interior ends of the thecal ^^'alls. This 

 ring held open, w^hile the perisarc Avas being compressed, a small, vacant space 

 sufficiently long for the commencement of the formation of the pyrite nodules. 



Didymograptus nanus Lapworth 



Plate 15, flg'ures 4, 5 



Didymograpsus gem in us Nicholson. Qiiar. Jour. Geol. Soc. 1868. 24:134, 



pl.5, %.8, 9 

 Didymograpsus gem in us Nicliolson {pars). Ann, and Mag. Nat. Hist. ser. 4. 



1870. 5:346, iig.6b 

 Did y m ogr a p tus in dent us var. nanus Lapworth. Quar. Jour. Geol. Soc. 1875. 



31:647, pi. 33, fig.7d ; pl.35, fig.4a-e 

 Didymograptus indent us var. nanus Elles. Quar. Jour. Geol. Soc. 1898. 



54:511 

 Didymograptus nanus Elles & Wood. Monogr. Brit. Grapt. pt 1. Pal. Soc. vol. 



for 1901. p.47, pl.4, fig.5a-h 



Description. Nema and primary disk not observed, Sicula slender, full 

 length not observed. Branches attaining a length of 22 mm or more, slender, 

 of uniform width (1 . 8 mm), angle of divergence approaching at first 90° but 

 soon becoming about 12" or less. Thecae numbering 10 to 12 in 10 mm, 

 inclined at 30°, rather long and narrow (ratio of length to width 3:1), in con- 

 tact for one half of their length in the mature portion of the branch. Aper- 



^ See his figures, pl.l, fig.18 and pl,3, fig.lO. In the explanation of the latter figure 

 they are specially mentioned. 



