306 NEW YORK STATE MUSEUM 



Elles and Wood have, from the distinctness of the growth lines in D 

 caduceus, concluded that its periderm was of considerable thickness 

 and suggested that the twisting of the rhabdosome so characteristic of that 

 species was due " to the stiff and wirelike nature of the polypary caused in 

 part by this thickened chitinous covering." The specimen of I), gu rl ey i 

 figured on page 105, which retains but the pyritous filling of the somatic 

 cavities, shows that in this species, which possesses a like twisted nature of 

 the branches, the periderm was indeed thick enough to give the branch a 

 regular tubular outline in spite of the double curvatures of the interior 

 thecal cavities. 



That in this species, at least, the crossing and recrossing of the 

 branches is due to the growth in a widening spiral is evidenced by two 

 facts, (1) that a branch will in one crossing be superjacent and in the 

 other subjacent and (2) that the thecae in the flattened specimens appear 

 in successive links of the figures described [see pi. 19, fig. 7] on opposite 

 sides of the branches, indicating that the branches were not originally 

 twisted in one plane, as they appear now but in several planes or in a spiral. 



Dicellograptus sextans (Hall) 



Plate 19, figure i 



Graptolithus sextans Hall. Pal. N. Y. 1847. 1: 273; pi. 74, fig. 3 

 G rapt oli thus sextans Salter. Quar. Jour. Geol. Soc. 1849. 5:17; pi. 1, fig. 10 

 Diplograptus ? sextans McCoy. Brit. Pal. Foss. 1855. p. 9 

 Graptolithus sextans Carruthers. Siluria. Ed. 4. 1867. p.6i, fig. 8 

 Graptolithus sextans Nicholson. Quar. Jour. Cieol. Soc. 1S68. 24: 134 

 Didymograptus sextans Nicholson. Ann. lV' Mag. Nat. Hist. Ser. 4. 1870. 



5-- 35 6 , P-9 

 D i c r an gr ap t u s formosus Hopkinson. Geol. Mag. 1870. 7:356: pi. f6, fig. 2 

 D i c r an g rap t u s sextans Hopkinson. Geol. Mag. 1870. 7:356; pi. 16, fig. 1 

 Dicr a 11 gr ap t us sextans Lapworth. Cat. West. Scott. Foss. 1876. p.6; pi. 3, 



fig. 78 

 Dicellograptus sextans Lapworth. Belfast Nat. field Club. Kep't & Proc. 



v. 1, pt 4- '877- A P X - Pi- 7. fig- 4 

 Dicellograptus sextans Linnarsson. Sver. Geol. Unders. Ser. C, no. 31. 1879, 



p. 1 si" 



