678 



NEW YORK STATE MUSEUM 



Fis. 73 Didymograptus 

 si mil is Hall ^ji. Very early 

 growth stage of rhabdosome. 

 Obverse view. Shows sicula and 

 first thecae. Deep kill x5 



rapidly from .8 mm in the proximal part to the maximal width (about 

 1.8 mm), which is maintained. First and second thecae originate near the 

 apex of sicula. Thecae numbering 9 to 10 in 10 mm, relatively short and 

 wide tubes ; width one third to one half of the length ; inclined at an angle 

 of about 30°, their outer margin straight or slightly concave, free for one 



third to one fourth of their length ; apertural margin 

 straight, forming with the axis of branch an angle of 

 about 120° (normal on axis of theca). 



Position and localities. Common in graptolite 

 beds 3 and 5 of the Deep kill section (zone with 

 D. bifidus) and very rare in the beds trans- 

 itional from this zone to that with Diplograptus 

 dentatus, on Mt Moreno near Hudson. The originals of the species 

 were also taken from the same zone (Phyllograptus anna zone), three miles 

 above the St Anne river in Canada. The form has not been reported from 

 other localities. 



Remarks. This species, which seems in America to be restricted to the 

 zone with Phyllograptus anna and D. bifidus, has, on account of 

 its short and broad thecae, been compared by Hall with D. Sagittarius 

 from the Normanskill shales. With associated congeners it has little 

 similarity, but reminds sometimes of young forms 

 of D. extensus, from which it can be distin- 

 guished by the less closely arranged and somewhat 

 differently shaped thecae. It is however extremely 

 similar to the Swedish species D. suecicus 

 Tullberg and D . d e c e n s Tornquist. From the 

 careful descriptions of the former species published by Tullberg [1880, p.43] 

 and by Tornquist [1901 ; p. 13], and of the latter by Tornquist [1891, p.l8] 

 it can be inferred that D. similis tallies in all important charactei's and 

 completely in the measurements with these two forms ; for their angle of 

 divergence is 180°, number of thecae 9 to 10 in 10 mm, the angle of inclination 

 30°. Also the form and length of the sicula and the form of the thecae are 



Fig. 74 Didymograptus 

 similis Hall sp. Young rhab- 

 dosome. Reverse view. Deep 

 kill. x6.5 



