lOi 



XEW YORK STATE MT'SKUM 



with their dorsal sides, but he considered the structure as analogous with that 

 of Diplograptus, assuming that the fronds of tlie latter were formed by the 

 coalescence of two raonoprionidian branches, and as a corollary he inferi-ed the 

 presence of a solid axis or virgula as in Diplograptus. The rhabdosomes of 





Kg. 94 Phy Uogvaptii s an frustif olius Hall, o View of a rliabdosome. etched out of 

 limestone. Seen liom the reverse (antisiuular) side. x4 h Sienlar end of rhabdosonie, seen from 

 the obverse (sicular) side. .\0 c Khabdosouie seen from the sicular end. Shows the apertures of 

 the sicula (in the center), of the first two thecae (tlanking the sicula), and of the earliest portions 

 of the branches. x6. d Transverse section through a rhabdosome, showing- the central coeno- 

 sarcal canal and its four longitudinal septa. x6 (Copies from Holm) 



Di23lograptus have now however been demonstrated to be not the result of the 

 coalescence of two branches, bnt of the budding of thecae of one series alter- 

 nately on opposite sides. It is, hence, evident that the two genera are of 

 entirely different structure. Subsequent observers, as Lapworth, Tullberg 

 and Tornquist, have all corroborated Hall's conception of Phyllograptus, and 

 Tullberg recognized the close relationship between Tetragraptus and Phyllo- 

 graptus, 



