GRAPTOLITES OF NEW YORK. PART 1 689 



for but one third of their length in mature parts of rhabdosonie. Outer 

 margins concave, apertural margins straight, normal on axis of theca ; one 

 half the width of branch; provided with long, straight spines, which 

 have a direction perpendicular to the axis of ^^ ^^ 



the branch. The dorsal wall of the branches **^^li'(^|^V'''^^nl^^^l|| 

 also provided with ])erpendicular spines, i 



which alternate with the apertui-al ones. Bp'''fiv'*p°ihn^ai'po%oS*of%m„Vh"°A"h- 



. , 7 7 7- -r»  1 **"' <l"^'"'"y 8t Mount Moreno. x4.5 



l^ositiov and localtty. Kare in the 

 shales with Diplograptxis dentatu:^ at Mt Moreno, 



RemarTcH. This species is at once distin- 

 guished from all other forms of Didymograptus 

 known to me, by its double row of spines. There 

 Fig. 85 Didymograptus spin- '^^s howcvcr been described a species of Tetra- 



08U8 sp. nov. Portion of distal part , ,rf\ i\ , /^ i ^ r^/^- 



ofbranch. Ashhiii quarry at Mount graptus (1. acauthonotus (xurlev, 1896. 

 Moreno. x4 o r \ j ' 



p.65), from the Beekmantown shales at Point 

 Levis, which exhibits the same spinous dorsal margin as the species in hand, 

 and which may indicate the line of descent of this Didymograptus [p.558]. 





Didymograptus bifidus Hall sp. 



Plate 15, figures 1-3 



Graptolith US bi f idns Hall. Canadian Organic Remains, decade 2. 1865. p.73, 



pl.l, fig.l6-18; pl.3, fig.9, 10 

 .Didymograptus b if idu 8 Nicholson. Quar. Jour. Geol. Soc. 1868. 24:136 

 Didymograptus bifidus Nicholson. Ann. and Mag. Nat. Hist. ser. 4. 1870. 



5:346, fig.7 

 Didymograptus bifidus Hopkinson & Lapworth. Quar. Jour. Geol. Soc. 1875. 



31:646, pl.33, fig.8a-e 

 Didymograptus bifid us (?) Brogger. Die sil. Etagen 2 and 3, etc. 1882. p.41 

 Didyra ograp tus bifi dus Herrmann. Geol. Mag. ser. 3. 1886. 3:15 

 Didy mograpt us b i f i d u s Ami. Geol. Sur. Can. Rep't, ser. 2. 1889. v.3, pt2, 



p.ll6k 

 Didymograptus bifidus Barrois. Ann. de laSoc. Geol. du Nord. 1892. 20:92 

 Didymograptus bifidus Guiiey. Jour. Geol. 1896. 4:295 



