G!»0 



NEW YORK STATE MUSEUM 



D i d y m o g r a p t u s bi f i d u s Elles. Quar. Jour. Geol. Soc. 1898. 54:511 

 Didjraograptus bifidus Elles & Wood. Monogr. Brit. Grapt. pt 1 . Pal. Soc. 



vol. for 1901. p.42, fig.26a and b 

 Didyiuograptus bifidus Ruedemann. N. Y. State Paleontol. An. Rep't. 1902. 



p.56G, 567 



Description. Nema and primary disk not observed. Sicula long and 

 slender, about 2 mm long. Branches dependent, rounded at their base, then 



straight; angle of divei^gence of first thecae 90° to 100°, 

 that of branches mostly varying between 20° and 25° ; 

 the branches attaining a length of 26 mm, expanding 

 gradually to a maximal width of 2.4 mm near the distal 

 growing ends, the latter showing a rapid contraction. 

 Thecae closely arranged, numbering 13 to 15 in 10 mm, 

 inclined at an angle of 45° in the mature and of about 

 30° in the proximal parts ; two to three times as 

 long as wide ; in contact for one half of their length 

 in the proximal and three fourths in the mature parts. 

 Apertural margin slightly concave, forming an angle of 

 135° with axis of branch. 

 Position and localities. In the Deep kill section common, but restricted 

 to graptolite beds 3 to 5, which represent the zone with Phyllograptus 

 anna and D. bifidus. Hall records it from Point Levis and from a 

 point 3 miles above the river St Anne; Gurley lists it [1896] as a fossil of 

 the ' Phyllograptus anna zone (St Anne zone), and also as occurring 

 in the Upper Levis zone (zone w^ith Diplograptus dentatus, etc.), 

 from which it is absent at the Deep kill. He I'eports it further from Nevada 

 [ibid. p.304]. According to Ami the species occurs in various localities in 

 the province of Quebec [1889, p. 11 6k]. In Great Britain this species is, 

 according to Lapworth, Elles and Wood, " more common than any other 

 member of the dependent series ; it occurs in great numbers at one particular 

 horizon, namely, in the Upper Arenig beds below the zone of D . m u re h i - 

 s o n i ". It is cited there from various localities in Ireland, Scotland, Eng- 



Fig. 86 Didymograp- 

 tus bifidus Hall sjj. Por- 

 tions of rhabdosomes en- 

 larged to show the form of 

 the thecae. Beep kill. x5.25 



