GRAPTOLITES OF NEW YORK, PART 1 701 



forced apart by some accident. Closer comparison shows however that the 

 thecae are less closely arranged and free for a greater portion of their length. 

 The dorsal part of the common canal is so strongly thickened in this 

 species that it appears like a solid rod, or virgula, suggesting in this feature, 

 as in the general form, an approach to Dicellograptus. 



Family coenograptidae nom. nov. 



SIGMAGRAPTUS ' gCU. UOV. 



Two principal ' branches, Avhich together form a sigmoidally curved rhab- 

 dosome. From these originate alternately on both sides, denticulate side 

 branches which do not bifurcate again. 



It is evident that this genus belongs to the coenograptids, its closest rela- 

 tionship being with Coenograptus itself, from which it differs only by the 

 arrangement of the branches on both sides of the principal branches instead 

 of on one side. It can however be noticed on well pi-eserved specimens of 

 Coenograptus gracilis that every second side })ranch l)euds across 

 the principal stem to the same side as the preceding branch, so that the origi- 

 nal arrangement of the branches of Coenograptus was probably the same as 

 that in Sigmagraptus. As Coenograptus retains the sigmoid curve of the two 

 principal branches and is a younger genus, it can be safely concluded that it 

 is directly derived from Sigmagraptus. The species of Sigmagraptus, here 

 described, has, as we have shown before, [p.565j a Goniogi-aptus as ancestor. 



The genus Pterograptus has the same alternating arrangement of the 

 branches as Coenograptus, but they are there distinctly pendent and sugges- 

 tive of a derivation from a Bryograptus with dependent branches. In Pleuro- 

 graptus the branches are given off alternately on both sides as in Sigmagraptus 

 but are provided again with tertiary branches. This structure could be readily 

 referred to that of Sigmagpaptus. The genus Amphigraptus, finally, which 

 was erected by Lapworth [1873 ] for forms like G r a p t o 1 i t h u s diver- 

 gens Hall, from the Quebec shales of Levis, is said to have the branches not 



^ Sigma, the letter Sigma, grapho to write. 



